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OULU 2013
A 607
Matti Kuokkanen
DEVELOPMENT OF AN ECO- AND MATERIAL-EFFICIENT PELLET PRODUCTION CHAIN—A CHEMICAL STUDY
UNIVERSITY OF OULU GRADUATE SCHOOL;UNIVERSITY OF OULU,FACULTY OF SCIENCE,DEPARTMENT OF CHEMISTRY
A 607
ACTA
Matti K
uokkanen
A C T A U N I V E R S I T A T I S O U L U E N S I SA S c i e n t i a e R e r u m N a t u r a l i u m 6 0 7
MATTI KUOKKANEN
DEVELOPMENT OF AN ECO-AND MATERIAL-EFFICIENTPELLET PRODUCTION CHAIN—A CHEMICAL STUDY
Academic dissertation to be presented with the assent ofthe Doctoral Training Committee of Technology andNatural Sciences of the University of Oulu for publicdefence in Wetteri-sali (Auditorium IT115), Linnanmaa,on 26 April 2013, at 12 noon
UNIVERSITY OF OULU, OULU 2013
Copyright © 2013Acta Univ. Oul. A 607, 2013
Supervised byDocent Toivo KuokkanenDoctor Tuomas Stoor
Reviewed byDocent Ari VäisänenDoctor Antero Moilanen
ISBN 978-952-62-0103-0 (Paperback)ISBN 978-952-62-0104-7 (PDF)
ISSN 0355-3191 (Printed)ISSN 1796-220X (Online)
Cover DesignRaimo Ahonen
JUVENES PRINTTAMPERE 2013
OpponentProfessor Lauri Sikanen
Kuokkanen, Matti, Development of an eco- and material-efficient pellet productionchain—a chemical study. University of Oulu Graduate School; University of Oulu, Faculty of Science, Department ofChemistry, P.O. Box 3000, FI-90014 University of Oulu, FinlandActa Univ. Oul. A 607, 2013
Abstract
According to the EU’s strategy and the corresponding Finnish national strategy on wastematerials, all kinds of waste must be utilised primarily as material (reuse, recycling) andsecondarily as energy, and at the lowest level of waste hierarchy is their disposal usingenvironmentally friendly methods. Today material efficiency is an essential topic in promotingsustainable use of natural resources, industrial by-products and waste material. The present goalproposed by the EU sets the target for the total proportion of renewable energy as high as 38% by2020 in Finland. Up to 20 million tonnes of waste wood biomass per year are left unused inFinland, mainly in the forests during forestry operations, because supply and demand do not meet.As a consequence of high heat energy prices, the looming threat of climate change, the greenhouseeffect and global as well as national demands to considerably increase the proportion of renewableenergy, Finland currently has a tremendous interest in increasing decentralised pellet productionalongside of large-scale factories.
The aim of this thesis is to promote the development of eco-, material- and cost-efficientNordic wood-based pellet production and utilisation of pellet bio-ash by means of chemicalresearch. Using Finnish wood (sawdust and shavings) as a model raw material, the totalfunctionality of a pilot-scale pellet facility combined with an extensive chemical toolbox wastested in this study to promote development of an eco-, material- and cost-efficient wood-basedpellet production chain. The chemical toolbox includes measurements of moisture content,density, heat value, mechanical durability and particle size distribution, TG analysis andelementary analysis, as well as new applications for pellet biodegradation using BOD OxiTopequipment and optical microscopic staining methods.
To improve the quality of pellets, considering the profitability of production and occupationalsafety factors (wood dust exposure, fire and explosion risk), it is profitable to use different bindingagents, especially industrial by-products and locally utilisable residuals. Thus, lignosulphonate,residual potato flour and potato peel residue were used and tested as model adhesive bindingagents. The results showed that binding agents increased the quality of pellets and changed theirinorganic characteristics, but did not have a significant effect on their calorimetric heat values.Lignosulphonate even increased the rate of production. To characterise different starch-containingbinding agents, a new specific optical microscopic staining method was developed and tested, andthe initial results are presented in this thesis.
Wood pellet ash has potential as a liming agent, in soil remediation, as a soil fertilizer, and ingranulated form, in new applications such as road construction and waste water purification.Valuable information about raw materials, binding agents and the pelletizing process is necessarywhen developing good-quality pellets—a prime biofuel—from non-utilised low-value and/ormoist biomass that has undergone a cost-efficient drying process. This way pellet production willhave more essential importance in energy policy, especially in the European forest belt.
Keywords: binding agent, bio-ash, biodegradation, bioenergy, biomass, eco-efficiency,forestry, pellet production, wood pellet
Kuokkanen, Matti, Eko- ja materiaalitehokkaan pellettituotantoketjun kehittä-miseen liittyvä kemiallinen tutkimus. Oulun yliopiston tutkijakoulu; Oulun yliopisto, Luonnontieteellinen tiedekunta, Kemian laitos,PL 3000, 90014 Oulun yliopistoActa Univ. Oul. A 607, 2013
Tiivistelmä
Vallitsevan EU:n sekä Suomen kansallisen lainsäädännön mukaan kaikenlainen jäte täytyy hyö-dyntää ensisijaisesti materiaalina (uudelleenkäyttö, kierrätys), toissijaisesti energiana ja jätehie-rarkiassa alimpana tasona on sen hävittäminen ympäristöystävällisin keinoin. Materiaalitehok-kuus on nykyään välttämätön aihe edistettäessä luonnonvarojen, teollisuuden sivutuotteiden jajätemateriaalien kestävää käyttöä. EU-strategian mukainen tavoite uusiutuvan energian osuudel-le kaikesta energiantuotannosta Suomessa on 38 % vuoteen 2020 mennessä. Jopa 20 miljoonaatonnia jätepuubiomassaa vuodessa jää käyttämättä Suomessa lähinnä metsänharvennustöidenyhteydessä, koska kysyntä ja tarjonta eivät kohtaa. Seurauksena korkeista lämpöenergiahinnois-ta, uhkaavasta ilmastonmuutoksesta, kasvihuoneilmiöstä sekä globaalisista ja kansallisista vaati-muksista lisätä uusiutuvan energian osuutta, Suomessa on viime aikoina noussut voimakas kiin-nostus lisätä hajautettua pellettituotantoa suurten pellettilaitosten rinnalle.
Väitöskirjan tarkoituksena on edistää ja kehittää pohjoismaista eko- ja kustannustehokastapuupellettituotantoa ja pellettibiotuhkan hyötykäyttöä kemiallisen tutkimuksen avulla. Käyttäensuomalaista puuta (sahanpurua ja kutterinlastua) malliraaka-aineina, tässä tutkimuksessa testat-tiin pilot-mittakaavan pellettilaitoksen toimivuutta yhdistettynä laajaan kemialliseen ”työpaket-tiin”, edistämään tulevaisuuden eko-, materiaali- ja kustannustehokkaan pellettituotantoketjunkehittämistä. Kemiallinen työpaketti sisältää kosteuden, tiheyden, lämpöarvon, mekaanisen kes-tävyyden ja partikkelikokojakauman määritykset, TG- ja alkuaineanalyysin, kuten myös uudetsovellukset pellettien ja niiden sideaineiden biohajoavuuden määrittämiseksi BOD OxiTop -lait-teistoilla sekä optisen mikroskooppivärjäysmenetelmän.
Pellettien laadun kohottamiseksi, ottaen huomioon myös tuotannon kannattavuuden ja työter-veydelliset ongelmat (puupölylle altistuminen, tulipalo- ja räjähdysvaara), on perusteltua käyt-tää sideaineita, erityisesti teollisuuden sivutuotteita ja paikallisesti hyödynnettävissä olevia jäte-materiaaleja. Täten lignosulfonaattia, jäteperunajauhoa ja perunankuorijätettä käytettiin ja testat-tiin liimaavina mallisideaineina. Tulokset osoittivat, että sideaineet nostivat pellettien laatua jamuuttivat niiden epäorgaanisia ominaisuuksia, mutta niillä ei ollut merkittävää vaikutusta määri-tettyihin lämpöarvoihin. Lignosulfonaatti lisäsi selvästi pelletoinnin tuotantonopeutta. Työssäkehitettiin pelleteille uusi spesifinen optinen mikroskooppivärjäysmenetelmä erilaisten tärkke-lystä sisältävien sideaineiden karakterisointiin ja ensimmäiset tulokset on esitetty tässä väitöskir-jassa.
Puupellettituhka on potentiaalinen kalkitus- ja maanparannusaineena, lannoitteena sekärakeistettuna uusissa sovelluksissa, kuten tierakentamisessa ja jäteveden puhdistuksessa. Arvo-kas informaatio raaka-aineista, sideaineista sekä pelletöintiprosessista on välttämätöntä kehitet-täessä tulevaisuudessa hyvälaatuisia pellettejä, ”priimaa” biopolttoainetta, hyödyntämättömästähuonolaatuisesta ja/tai kosteasta biomassasta, joka on ennen pelletointia käynyt läpi kustannuste-hokkaan kuivausprosessin. Täten voidaan olennaisesti lisätä pellettituotannon merkitystä ener-giapolitiikassa, erityisesti Euroopan metsävyöhykkeellä.
Asiasanat: bioenergia, biohajoavuus, biomassa, biotuhka, ekotehokkuus, metsätiede,pellettituotanto, puupelletti, sideaine
7
Acknowledgements
This work was mainly carried out at the Department of Chemistry, Laboratory of
Physical Chemistry, and at the Fibre and Particle Engineering Laboratory at the
University of Oulu. This research was financially supported by the University of
Oulu, Vapo Inc., Formados Inc., Naturpolis Oy, Kuusamon Energia- ja
Vesiosuuskunta, The Fortum Foundation, the Tauno Tönning Foundation and
Oulun Läänin Talousseuran Maataloussäätiö. All of these founding bodies are
greatly acknowledged. In addition, I would like to thank the following people
who have helped and encouraged me during this work:
To begin with, I would like to express my deepest gratitude to my
supervisors, Docent Toivo Kuokkanen (Ph.D.), Tuomas Stoor (Dr. Tech.) and
Jouko Niinimäki (Dr. Tech, Professor), for providing me with the opportunity to
work with this interesting subject. Toivo, Tuomas and Jouko have always
instructed me patiently and given me very important advice in relation to my
research. Special thanks go to Hanna Prokkola (M.Sc.) for her valuable
contributions with the experimental work. Also the co-authors of my articles, Veli
Pohjonen (Dr. Tech, Professor), Teemu Vilppo (M.Sc., Tech.), Jaakko Larkomaa
(M.Sc., Tech.), Leena Siltaloppi (M.Sc.), Hannu Nurmesniemi (Ph.D., Docent)
and Risto Pöykiö (Ph.D., Docent), are warmly acknowledged. In addition, I
would like to thank Suomen Ympäristöpalvelu Oy and Ilkka Välimäki (M.Sc.) for
the analyses of the bio-ash samples and Formados Inc. and especially CEO Kyösti
Nevala, who were involved in the full industrial scale manufacture of pellets at
their pellet plant in Kuusamo.
I also wish to thank Mr. Risto Ikonen from the University of Eastern Finland,
Ph.D. Juha Koskela, M.Sc. Juhani Kaakinen, M.Sc. Riitta Raudaskoski, M.Sc.
Henna Jokinen and M.Sc. Tommi Kokkonen from the University of Oulu and
M.Sc. Ritva Imppola, M.Sc. Mikko Aalto and M.Sc. Tech. Heikki Takalo-Kippola
from Oulu University of Applied Sciences and Ms. Reetta Kolppanen from the
Finnish Forest Research Institute Metla for their scientific co-operation and
Kotirannan Vihannesjaloste Oy and CEO Timo Jaakola, Evijärven Peruna Oy,
Liperin Juurespakkaamo Oy and Pinifer Oy for their binding agent co-operation.
Lastly, I want to thank my fellow students and especially my friends. My
warmest gratitude goes to my parents Vuokko and Toivo and to my brothers Mika
and Ville and their families. They all have encouraged and supported me during
my studies and reminded me about the significance of education and hard work.
Oulu, 2013 Matti Kuokkanen
8
9
Abbreviations and definitions
EU European Union
TG Thermogravimetry
BOD Biological oxygen demand
ICCE International Congress of Chemistry and Environment
ICSW International Conference on Solid Waste Technology and
Management
LCA Life cycle assessment
CEN/TS European Committee for Standardisation / Technical Specifications
PM Particulate matter
CHP Combined Heat and Power
REACH Registration, Evaluation and Authorisation of Chemicals
EC European Council
PPW Potato peel waste
SFS-EN Finnish Standards Association
d.w. Dry weight
ICP-OES Inductively coupled plasma optical emission spectrometry
VOC Volatile organic compound
BET (Brunauer, Emmett and Teller) Theory
SEM Scanning electronic microscopy
ISO International Organization for Standardization
BMA Biomaterial moisture analyser
DIN Deutsches Institut für Normung
ASTM American Society for Testing and Materials
EEC European Economic Community
OECD Organisation for Economic Co-operation and Development
BOD28 Biological oxygen demand in 28 days
ThOD Theoretical oxygen demand
TA Thermoanalytic
DSC Differential scanning calorimetric
EPA Environmental Protection Agency
L/S Liquid to solid ratio
VNa Valtioneuvoston asetus (in Finnish) (Government Decree)
PIMA Asetus pilaantuneista maista (in Finnish) (Assessment of Soil
Contamination and Remediation Needs)
PCC Precipitated calcium carbonate
10
PBPM Potential bioavailability percent
R&D Research and development
FINAS Finnish Accreditation Service
EAKR Euroopan Aluekehitysrahasto (in Finnish) (European Regional
Development Fund)
MC Moisture content
BOD7 Biological oxygen demand in 7 days
TOC Total organic carbon
COD Chemical oxygen demand
GC-MS Gas chromatography-mass spectrometry
PAH Polycyclic aromatic hydrocarbon
wt% Weight percent
vol-% Volume weighted percentage
LOI Loss on ignition
NV Neutralising value
BCR Community Bureau of Reference
SPE Suomen Pellettienergiayhdistys ry (in Finnish) (Finnish Pelletenergy
Association)
RME Rape methyl ester
11
List of original papers
This thesis is based on the following articles, which are referred to in the thesis by
their Roman numerals:
I Kuokkanen M, Kuokkanen T, Stoor T, Niinimäki J & Pohjonen V (2009) Chemical Methods in the Development of Eco-efficient Wood-based Pellet Production and Technology, Waste Management & Research 27: 561–571.
II Kuokkanen M, Vilppo T, Kuokkanen T, Stoor T & Niinimäki (2011) Additives in Wood Pellet Production – A Pilot-Scale Study of Binding Agent Usage, BioResources 6 (4): 4331–4355.
III Kuokkanen M, Prokkola H, Larkomaa J, Stoor T, Siltaloppi L & Kuokkanen T (2010) Specific Staining and Optical Microscopy – a New Method for Characterisation of Starch-containing Wood Pellets, Special Issue of Research Journal of Chemistry and Environment, Proceedings of ICCE-2009: 311–317.
IV Kuokkanen M, Kuokkanen T, Nurmesniemi H & Pöykiö R (2009) Wood pellet ash – a potential forest fertilizer and soil conditioning agent (a case study), The Journal of Solid Waste Technology and Management, Proceedings in ICSW 2009: 659–667.
V Kuokkanen M & Kuokkanen T (2009) Puu- ja turvepellettien sekä hakkeen lämpökeskus- ja pienpoltossa syntyvien tuhkien hyötykäyttöön liittyvä tutkimusraportti. University of Oulu, Report Series in Chemistry, Report No. 74.
The present author was the primary author and corresponding author of all articles
I–V of this dissertation. The design of analyses, the bulk of the experimental work
as well as the analysis of the results related to Articles I and III were carried out
by the present author. The design of the research and the analysis of the chemical
results as well as partly the experimental work of Article II were the work of the
present author. The design of the research and the analysis of the results but only
partly the experimental work of Articles IV and V were carried out by the present
author.
In addition, the results of the article “Kuokkanen, M., Vilppo, T., Kuokkanen,
T., Stoor, T. & Koskela, J. (2011) Pilot-mittakaavainen sekä kemiallinen tutkimus
eräiden lisäaineiden käytöstä puupellettituotannossa, EkoPelletti, Raportit ja
julkaisut, 21 pp. http://www.oamk.fi/hankkeet/ekopelletti/docs/pilot-
mittakaavainen.pdf” are discussed in this thesis. The article was jointly written by
the present author and co-author M.Sc. Teemu Vilppo.
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13
Contents
Abstract
Tiivistelmä
Acknowledgements 7 Abbreviations and definitions 9 List of original papers 11 Contents 13 1 Introduction 15 2 Aims of the study 23 3 Materials and methods 25
3.1 Materials ................................................................................................. 25 3.1.1 Lignosulphonates .......................................................................... 25 3.1.2 Potato starch ................................................................................. 26 3.1.3 Potato peel residue ........................................................................ 27 3.1.4 Wood pellet ash ............................................................................ 28
3.2 Research methods ................................................................................... 28 3.2.1 Moisture content measurement ..................................................... 29 3.2.2 Density measurements .................................................................. 30 3.2.3 Calorific heat measurement .......................................................... 32 3.2.4 BOD measurements ...................................................................... 33 3.2.5 Mechanical durability measurement ............................................. 36 3.2.6 Microscopic structure analysis ..................................................... 36 3.2.7 TG analysis ................................................................................... 37 3.2.8 Elemental analysis ........................................................................ 38 3.2.9 Particle size distribution ............................................................... 39 3.2.10 Sequential leaching procedure ...................................................... 39
4 Pelletizing machinery 43 4.1 Delivery of raw material and receiving stock ......................................... 43 4.2 Drying unit and pre-treatment of raw material ........................................ 43 4.3 Pelletizing machine ................................................................................. 44 4.4 Pelletizing field tests ............................................................................... 47
5 Experimental work 51 6 Results and discussion 53
6.1 Moisture content ..................................................................................... 53 6.2 Calorimetric heat (I,II) ............................................................................ 54 6.3 BOD measurements (I,II) ........................................................................ 57
14
6.4 Binding agent usage (I, II)....................................................................... 60 6.5 Visual analysis of pellet sample images (I, II, III) .................................. 64 6.6 TG analysis ............................................................................................. 68 6.7 Particle size distribution (II) .................................................................... 71 6.8 Wood-pellet-based bio-ash (II,IV,V) ....................................................... 73
6.8.1 Utilisation potential of bio-ash ..................................................... 82 7 Conclusions 85 8 Future work 89 References 91 Original papers 99
15
1 Introduction
It is estimated that 15–20 million tonnes of waste wood biomass per year are left
unused in Finland, mainly in the forests during forestry operations, because
supply and demand do not meet. According to the present Finnish strategy on
waste materials—in accordance with the corresponding EU strategy—all kinds of
waste must be utilised primarily as material (reuse, recycling) and secondarily as
energy, and if neither of these utilisation methods is possible, they can be
disposed of using ecologically beneficial methods. Today material efficiency is an
essential topic in promoting sustainable use of natural resources, industrial by-
products and waste material, in agreement with the principles of sustainable
development and LCA (life cycle assessment). Recycling and exploitation of
waste are perceived as also belonging to the concept of material efficiency.
Formation of waste can be considered an indicator of ineffectiveness in industry
[1,2,3]. Eco-efficiency means optimal combined financial and ecological use of
natural resources and can be described as a management strategy of doing more
with less. It is achieved by increasing product or service value, optimizing the use
of resources and reducing environmental impact. In accordance with these goals
and as a consequence of high heat energy prices, the looming threat of climate
change, the greenhouse effect and global as well as national demands to
considerably increase the proportion of renewable energy, there are currently
goals of expanding the raw material base in Finland and also substantially
increasing pellet production. As a part of this European objective to increase the
eco- and cost-efficient utilisation of bio-energy from the European forest belt, the
aim of our research group is—by means of multidisciplinary research, especially
through chemical methods—to promote the development of Nordic wood-based
pellet production in both the qualitative and quantitative sense. It is noteworthy
that wood-based pellets are classified as an emission-neutral fuel, i.e. they are free
from emission trading in the EU.
Wood pellets, taking into consideration the large amounts of utilisable waste
wood material worldwide, have been developed and patented surprisingly late.
The first patent for wood pellet manufacturing was registered in 1976 in the USA
in the aftermath of the global oil crisis. Wood pellets are short cylindrical pieces
(usually with a diameter of 6 to 10 mm and a length of 10 to 30 mm) which are
produced mechanically by compressing a uniform material that has first passed
through a hammer mill or mills to provide a homogeneous particle size. This mass
is fed to a press where it is pressed through a die with holes of the size required.
16
Typically the diameter of pellets is 8 mm in the Nordic Countries, 6 mm in
Central Europe [4] and sometimes 15 mm and up to 25 mm according to CEN/TS
14961 [5]. The high pressure and friction of the press causes the temperature of
the wood to increase significantly, slightly plasticizing the lignin in a temperature
range of 100 to 130 °C [6] and forming a natural 'glue' that holds the pellet
together as it cools. At the same time pelletizing increases the density of the
biomass, which is especially important logistically when low-density biomass is
pelletized. The initial raw materials used in pellet production are sawdust and
cutter shavings, which are by-products of the mechanical forest industry. The raw
material used in pellet production is typically stem wood. It has been concluded
that utilisation of high-quality wood fuel, such as wood pellets produced from
natural, uncontaminated stem wood, would generate the least particulate matter
(PM) emissions compared with other wood fuel types [7]. Taking into
consideration the large amounts of utilisable residual wood materials worldwide
and the high price of crude oil, global interest in developing pellet technologies
and increasing wood-based energy production has grown in recent years. The
present goal suggested by the EU sets the total proportion of renewable energy as
high as 25% by 2020. As a consequence of the EU strategy, this proportion must
grow to even 38% in Finland by 2020.
The biggest biomass potential in the EU region lies in forested areas (see Fig.
1). It is worthwhile noticing that these are the most rural areas and there are no
other methods available there for utilising this biomass potential, because supply
and demand do not meet in these areas where biomass is growing perhaps more
than ever before (biomass potential in more detail in paper I). It is notable that if
the pellet market increases in the future, it will have much more financial
importance to rural areas than its production volume indicates, because pellet
prices are much higher than wood chip prices. Correspondingly, it has a stronger
employment factor for these areas than wood chip production. On the other hand,
in Finland these are the same areas that have potential in the strongly growing
mining business, especially in the Lapland, North Karelia and Kainuu areas. This
is important because wood pellets could be a partial source of energy in these
mines as the prime biofuel. Altogether this is a matter of cost-efficiency. Forest
economists have calculated that with present costs the maximum distance for
profitable transport of forest chips, sawdust and shavings in Finland is 100 km,
for round wood it is approximately 1000 km, but for wood pellets transported by
sea the figure is, however, even 5000 km because sea transportation is the
cheapest way to transport. Shorter-distance transports of biomass in land areas are
17
carried out by trucks and trains, which are more expensive, but faster on the other
hand.
Fig. 1. Forest map of the European region [8].
These calculations, in conjunction with the general demands to increase
utilisation of renewable energy as opposed to fossil fuels, explain why there is
strong pressure to increase production of wood-based pellets in particular (for
more details, see paper I). This biomass material has potential for utilisation by
pelletizing it, in both large-scale factories and decentralised small-scale
manufacture. A map of European pellet plants is presented in Fig. 2. This figure
combined with the information of Fig. 1 predicts that the areas with the biggest
growth potential in pellet production are the Northern countries and Russia.
18
Fig. 2. A map of European pellet plants [9].
The number of industrial-scale pellet factories in Finland today is about 25, with a
total annual production of about 300,000 tonnes in recent years, owing to the
narrowness of the raw material base. However, total production capacity is much
larger, even nearly 1 million tonnes per year [10]. Pellet production started in
19
Finland in 1998; the first plant was established in Vöyri, with annual production
of 10,000 tonnes. In 2001 total annual production in Finland was about 80,000
tonnes and in 2005 it was even about 200,000 tonnes, which shows the rapid
development in pellet production [10,11]. The aim in the future is to utilise more
low-value and/or moist biomass to expand the base of wood pellet raw material,
especially logging residue from thinning. For example, if only 10–15% of
unusable waste wood biomass from forestry operations could be utilised, it would
raise the annual wood pellet production capacity significantly, being about 2–3
million tonnes. However, there is reason to point out that the use of poor-quality
raw material requires pre-treatment methods such as drying, which raises the cost
as energy consumption increases. Nevertheless, since the purchase prices of these
poor-quality biomasses are very low, sometimes almost free, it doesn’t increase
the total cost of pellet production significantly.
To improve the quality of pellets, commercial and industrial by-product
materials are commonly used as binding agents in pellet production [4,6,12].
Also, considering the profitability of production and occupational safety problems
(wood dust exposure, fire and explosion risk, coherence, etc.), it is practical to use
binding agents. The use of binding agents increases the cost of production, but on
the other hand the increase in cost can be added to the price of a high-quality
product. Pellets are subject to mechanical loads and environmental factors during
production, transportation and storage, i.e. pellets are pumped into storage silos,
transported by truck or ship and delivered to the customer. During all these steps
there is the risk of mechanical failure of the pellet, resulting in fine particle and
dust formation, which have shown to have harmful effects on human health
[13,14]. It has also been noticed that pellets may decompose during storage,
forming gases such as carbon monoxide and hexanal, which are hazardous to
human health. Typically this happens with pellets made from softwoods.
Decomposition of pellet biomass may produce large quantities of carbon
monoxide and hexanal during pellet storage [15,16,17].
A partial aim of the present research is to find and test some eco- and cost-
efficient materials for binding purposes and thus improve the competitiveness of
pellet products as an alternative in energy production. Therefore, various locally
tailored binding agents, such as certain industrial by-products and residuals,
especially starch-containing waste materials, are currently under study. In this
thesis three potential binding agents, lignosulphonate, residual potato flour and
potato peel residue, were selected primarily because of their local availability and
secondly for their properties. Potato peel residue represents waste from important
20
production sector in Finland and its material-efficient utilisation applications are
under wide study (bioethanol, biogasification for CHP production, animal food,
pellet production, etc.). Utilisation of residual potato flour as binding agent
represents modern material-efficient use of waste or industrial by-product.
Although potato flour is commonly used commercial binding agent in pellet
plants, we focused our study on the use of residual material in details.
Lignosulphonate proved as functional binding agent in our preliminary
experiments and represents the use of industrial by-product or commercial
product from wood pulp production. However, there were no chemical studies
concerning the use of lignosulphonate as pellet binding agent.
Due to the low density of the biomass, pelletizing is required for efficient
transportation and utilisation in some cases, for example in long-distance
transports. To ensure a continuous supply of fuel and to maintain consumer
confidence in biofuel products, the pellet industry requires a broader raw material
base and less dependency on sawmill stem wood by-products [10]. Small-scale
pellet facilities are required in order to decrease low-density transportation from
outlying areas to centralised consumer markets and also for small-scale heating
applications. To support small-scale industry, independent research is required to
provide information on how to produce pellets that meet strict quality criteria. For
lower-quality raw material, this requires extensive knowledge about locally
usable and available binding agents that provide the means to produce high-
quality pellets.
As described earlier, according to the present EU and Finnish strategies on
waste materials, all kinds of waste must be utilised primarily as material (reuse,
recycling) and secondarily as energy, and if neither of these utilisation methods is
possible, they can be disposed of using ecologically beneficial methods. The
present goals of the Finnish waste strategy are given in the national waste plan
approved by the Finnish Council of State on the 10th of April 2008 [18] and in the
new waste law, approved on the 17th of June 2011; the changes took effect on the
1st of May 2012 [19].
The intent is to reach these goals by 2016. The key goals of this national
waste plan are to: 1) reduce waste, 2) increase waste material recycling and
biological reuse, 3) increase combustion of non-recyclable waste and 4) ensure
harmless treatment and final placement of waste. In addition, regional waste
plants supplement the above-mentioned national waste plan. According to the EU
and national waste strategies, the primary goal for ash is its utilisation as a
material. A major change in the status of bio-ash has happened during the last
21
years, as earlier the main treatment method for all kinds of ash was to place them
in landfill and dumping places—all kinds of ash were considered to be unusable
waste, even problematic hazardous waste. Wood and wood-based pellet ashes are
bio-ashes which are generated as by-products of biomass combustion or
gasification processes in heat and power generation. With the change in the
EU/Finnish waste strategy, there is currently a tremendous interest in Finland to
substantially increase the utilisation of bio-ash, develop new applications and
develop new bio-ash-based products, especially granulated bio-ash-based
products. Utilising pellet ash instead of disposing of it in landfills—with
increasing landfill costs and a new waste tax on all industrial landfills—will
increase the profitability of the whole chain of bioenergy production and thereby
that of pellet production.
According to the REACH regulation (1907/2006/EC), all chemical
substances of which at least one tonne per year is produced or imported must be
registered [20]. So, if ash materials are placed in landfills and dumps, they need
not be registered but they belong under waste taxation. Registration of European
wood-based ashes is just now underway, including altogether 71 ash producers in
the beginning of December 2010. This broad consortium indicates the importance
and topicality of the potential utilisation of bio-ashes and also wood pellet ashes.
For all the reasons presented above, pellet ash has been studied and treated widely
in this doctoral thesis.
22
23
2 Aims of the study
This study consists of five articles (I–V) in which chemical analyses are used for
the development of eco-, material- and cost-efficient pellet production and
technology. Pellet production and therefore scientific—especially chemical—
research on pellets are a very new topic area and thus there is also a strong
demand for information concerning pellets and pellet research. Roughly speaking,
as far as we know, there are no studies concerning the chemical perspective of the
pelletizing issue.
The first aim of this dissertation was to introduce general information on
pelletizing and chemical research on pelletizing development for scientific
purposes and the bioenergy sector. This topic is dealt with especially in Articles I
and II. In addition, this topic has been discussed in oral presentations at many
scientific forums. The following presentations are presented below:
1. M. Kuokkanen: Chemical methods in the development of eco-efficient pellet
production and utilisation of wood ash, HighBio-seminar, Luleå, Sweden
12.5.2009. URL: https://ciweb.chydenius.fi/project_files/HighBio-
Projektiseminaari%20120509/Matti%20Kuokkanen%20HighBio-
Lulea%201205-09%20Chemical%20methods.pdf
2. M. Kuokkanen: Ekotehokkaan puupohjaisen pellettituotannon kehittäminen,
EnePro-conference, Oulu, Finland 3.6.2009. URL: http://nortech.oulu.fi/
EnePro/Kuokkanen_EnePro.pdf
3. M. Kuokkanen: Chemical methods in the development of eco-efficient wood
pellet production – Specific staining and optical microscopy, ICCE 2009
conference, Ubonratchathani, Thailand 21.1.2010.
The second aim was to test and develop new chemical methods for use in pellet
research, in addition to methods used in the recent pellet standards. These new
methods, BOD measurement and an optical staining procedure, are included in
the chemical research toolbox which is presented and discussed in Articles I, II
and III. As far as we know, these methods have not been presented in pellet-
related literature earlier.
Today material efficiency is an essential topic in promoting sustainable use of
natural resources. According to the present Finnish strategy on waste materials,
and in accordance with the corresponding European strategy, all kinds of waste
must be utilised primarily as material (reuse, recycling) and secondarily as energy,
and if neither of these utilisation methods is possible, they can be disposed of
24
using ecologically beneficial methods. The current waste hierarchy is presented in
Fig. 3. Thus, the goals of utilising biomass as pellets, waste adhesive materials as
binding agents in pellet production and bio-pellet ash for different applications,
all represent the top global goals in utilisation of waste materials and industrial
by-products.
Fig. 3. Waste hierarchy [21].
Material efficiency in terms of industrial ecology principles is dealt with in all the
articles I-V, because pellets and some of their binding agents are recycled waste
materials. Respectively, utilisation of especially bio-ashes is a hot topic now in
the Northern countries, where ash materials are widely available. However, heavy
metal concentrations may limit the usability of the ashes. Keeping in mind the
utilisation potential, the chemical properties of wood pellet ashes are discussed in
Articles IV and V.
25
3 Materials and methods
3.1 Materials
The effect of binding agents on the operation of a pilot-scale pellet facility was
studied using Finnish conifer bark-free stem wood as model raw material and
lignosulphonate, residual potato flour and potato peel residue as binding agents.
The aim of this thesis was also to study the effect of binding agents on the various
properties of wood-based pellets.
3.1.1 Lignosulphonates
Lignosulphonates, or sulphonated lignins, are water-soluble anionic
polyelectrolyte polymers. They are by-products of wood pulp production that
employs sulphite pulping [22]. Most delignification in sulphite pulping involves
acidic cleavage of ether bonds which connect many of the constituents of lignin.
Electrophilic carbocations produced during ether cleavage react with bisulphite
ions (HSO3-) to give sulphonates [23].
The primary site for ether cleavage is the α-carbon (carbon atom attached to
the aromatic ring) of the propyl (linear three-carbon) side chain. The following
diagrams do not detail the structure, since lignin and its derivatives are complex
mixtures—the purpose is to provide a general idea of the structure of
lignosulphonates (Fig. 4). The groups labelled “Q” may be a wide variety of
groups found in the structure of lignin. Sulphonation occurs in the side chains, not
in the aromatic rings, like in p-toluene sulphonic acid [22].
26
Fig. 4. Preparation and structure of lignosulphonate [22].
The sites of potential cross-linking reactions in lignosulphonates are at
unsubstituted ring positions of ortho- to free phenolic hydroxyls, as in
phenolformaldehyde condensations, or direct reactions of free phenolic hydroxyls
with ether-forming agents, as in epoxide condensations. In either case, phenolic
hydroxyls are necessary, and the primary bond formed is very stable, a necessary
criterion for adhesive durability. Thus, the phenolic hydroxyl content of
lignosulphonates may be an important criterion of its potential reactivity [22,23].
3.1.2 Potato starch
Starch is a carbohydrate consisting of a large number of glucose units joined
together by glycosidic bonds. This polysaccharide is produced by all green plants
as energy storage. It is contained in such staple foods as potatoes, wheat, corn,
rice and cassava. Pure starch is a white, tasteless and odourless powder that is
insoluble in cold water and alcohol. Starch consists of two types of molecules:
linear, helical amylose (Fig. 5) and branched amylopectin (Fig. 6). Depending on
the plant, starch generally contains 20 to 25% amylose and 75 to 80%
amylopectin [24]. For potatoes, the values are 25% amylose and 75%
amylopectin. When starch is dissolved in warm water, it can be used as a
thickening, stiffening or gluing agent.
27
Fig. 5. Structure of amylose [25].
Fig. 6. Structure of amylopectin [25].
3.1.3 Potato peel residue
Potato peel waste (PPW) is a starch-rich residue [26] from the potato peeling
process. Disposal of this process residue is problematic due to its extremely high
organic content, which readily decomposes but has high nutritional value as
animal feed [27]. The residue is also rich in nutrients, especially potassium. Using
it in pellet production without a sterilisation process poses a significant disease
risk to potato plantations and an environmental risk if it is not properly processed
[28].
28
3.1.4 Wood pellet ash
Is it generally known and numerous studies have shown [29,30,31,32] that many
factors affect the composition and amount of bio-ash. The physical and chemical
qualities of ash vary significantly depending on factors such as the ratio of the
fuels burned, tree species, geographical location, growing site, climate, tree
components (e.g. bark, wood, leaves) and the season of cutting. Other factors
which also affect the characteristics of the ash are the size and age of the tree, the
manner of collection and storage and the burning technique, such as the
combustion temperature and type of boiler [33]. As examples, two samples of
wood-pellet-based bio-ash from Kuusamo and Jyväskylä are considered in this
summary part of the thesis. The corresponding results for the other analysed bio-
ashes as well as other ash results are presented in detail in papers II, IV and V.
Ash sample 1 was taken from a grate-fired boiler in a small-scale district
heating plant in Kuusamo, Eastern Finland (papers IV and V). The plant has a
100-kW boiler, and 100% of the energy produced by the boiler originates from
the incineration of commercial wood pellets. The ash—technically speaking
bottom ash—was sampled as dry from the outlet of the boiler. The boiler
incinerates ca. 40 tonnes (d.w.) of commercial wood pellets per year. The
commercial wood pellets incinerated in the boiler were made from cutter shavings
and sawdust (ca. 80–90% pine and ca. 10–20% spruce) originating from the
mechanical wood-processing industry in the Kuusamo region.
Ash sample 2 was taken from a grate-fired boiler in a small-scale district
heating plant at the Jyväskylä airport, Central Finland (paper V). The plant has a
700-kW boiler, and 100% of the energy produced by the boiler originates from
the incineration of commercial wood pellets. The ash is the mixture of bottom ash
and fly ash and it was sampled from the ash silo of the boiler, which was emptied
1.5–2 months earlier, so the compilation sample represents an approximately 1.5-
month burning period. The boiler incinerates ca. 330 tonnes (d.w.) of commercial
wood pellets per year.
3.2 Research methods
The most important chemical methods used in this pellet research are shown in
Table 1 and are described in following chapters.
29
Table 1. Chemical toolbox for pellet production and development research used in this
work.
Measurement Equipment Purpose of determination
Moisture content Heat oven and scales
Automatic Senfit method
Water content
Density Solid: Laboratory compacted bulk
density, SFS-EN-13040
Liquid: Densitometer
Compactness of materials
Heat value measurements Bomb calorimeter Energy content
BOD tests BOD OxiTop® equipment
for liquid and solid phases
Biodegradation/loss of material
Dust content, strength
properties
Vibrator, sieve analysis Mechanical stability of the pellet
Structure analysis Staining reagents and microscope Information about pellet structure
and cross-linking mechanisms
TG analysis Thermogravimetric analyser Volatilisation of water and VOCs
Elementary analysis ICP-OES Heavy metals and/or nutrients of
pellet and ash
Particle size distribution Laser diffraction particle size
analyser, image analysis
Information about particle size
distribution in wood and ash
In addition, BET for specific surface area determination and SEM (Scanning
electronic microscopy) have recently been used in studies of wood-based ash, but
these methods were not used in this thesis.
3.2.1 Moisture content measurement
Moisture content was measured according to the ISO 589 [34] and CEN/TS
14774-1 standards [35]. According to ISO 589, the samples were dried overnight
to a constant mass (16–24 h) in a drying oven at 105 ± 2 °C. After drying, the
samples were placed in cooling desiccators and weighed. The moisture content
could then be calculated based on the decrease in mass during the drying stage.
According to CEN/TS 14774-1, the samples were dried to a constant mass (up to
24 h) in a drying oven and immediately weighed when hot.
Measurement of moisture content according to the drying/weighing method
presented above is very simple and reliable and requires no expensive equipment,
and therefore it is generally used for biomass studies and also in trading of raw
materials of bioenergy. However, this method includes many disadvantages such
as: i) it is time-consuming (the temperature is only a little higher than temperature
30
of the water), ii) the amounts of samples are relatively small (the sample layer
cannot be very thick because of the “free” water vaporisation), iii) VOC
compounds can be vaporised during the drying process and iv) the measurement
is not reliable if the moisture is contained within the cells. For these reasons many
practical and automatic methods for determining the moisture content of biomass
quickly but faithfully have been developed and tested during the last decade.
During this thesis work automatic rapid analysis methods were generally used to
measure the moisture content of raw materials and pellets in full-scale pellet
factories. These methods are based on CEN/TS 14774-1 and they generally
measure the increase in electrical conductivity produced by water in the sample.
It is noteworthy to observe that the importance of using these automatic
methods increases all the time with increasing international trading of renewable
energy. The measurements of moisture content in this study were all carried out
according to the ISO 589 and CEN/TS 14774-1 standards, but later on some small
automatic devices have been fitted and tested in our laboratory. A new Finnish
patented invention—a biomaterial moisture analyser (BMA)—seems to be very
promising for large-scale biomass trading [36] and is therefore briefly described
below.
Measurement with a BMA is done by using a patented microwave resonator
structure [36]. Detecting the fraction of water in a vast variety of different
materials is based on the unique electrical properties of water in a high-frequency
electromagnetic field. The key parameter—the permittivity of water—differs
significantly from most other materials, changing the response of the resonator.
Microwave resonator sensors are commonly used in measurements of low water
content (< 30%). BMA measurement utilises a new method where a resonating
wave mode is selected to be useful also with very high water contents. The
BMA’s fully digital signal processing uses multi-parameter calculation. Special
algorithms are used to compensate for density changes [36].
3.2.2 Density measurements
The densities of liquids (binding agents in a liquid phase) were measured at room
temperature with a common procedure using an AP Paar DMA 40 digital
densitometer, based on an oscillating U-tube principle where the oscillating
frequency is dependent on the density of the liquid sample, not on its
composition.
31
Pellet bulk (solid phase) densities were measured using equipment with a
measuring cylinder of 1 L and a weight of 650 g (8 g cm-2) (Fig. 7), according to
standard SFS-EN-13040 [37].
Fig. 7. Bulk density test cylinder. 1) Plunger, 2) Supported sieve, 3) Funnel 4) Collar 5)
Test cylinder, d = diameter: 100 mm ± 1 mm, h = height: 127 mm ± 1 mm, d1 = diameter:
95 mm ± 1 mm [37].
32
3.2.3 Calorific heat measurement
Calorimetric heat measurements and calculation of net/gross calorific heat values
were performed according to standards DIN 51900 [38], ISO 1928 [39] and
ASTM D240 [40]. In this study calorific heat was determined for pellet samples
and binding materials with an IKA C5000 calorimeter at the Finnish Forest
Research Institute (Metla) Kannus Research Unit and with an IKA C200
calorimeter at the Department of Chemistry, University of Oulu (see Table 2,
comparison test). The weighed portion of the analysis sample of the solid biofuel
was burned in a high-pressure oxygen bomb in a bomb calorimeter under
specified conditions. The effective heat capacity of the calorimeter was
determined in calibration experiments by burning certified benzoic acid under
conditions similar to those specified in the certificate. The heat released from the
burned sample was observed with a digital thermometer. The heat values were
determined with at least duplicate measurements. The calorimeter constant C was
determined using equation (1):
C = (ΔH x m + Q) / ΔT1 (1)
where ΔH is the heat of combustion of the calibration compound (ΔH = 26.44
kJ/g for benzoic acid), m is the mass of the calibration compound [g], Q is the
heat of combustion of the fuse igniter (50 J) and ΔT1 is the temperature change in
calibration. After determination of the calorimeter constant C, the gross calorific
values of the air dry (i.e. “air wet”) samples, Qgr,ad [kJ/g], were calculated using
equation (2):
Qgr,ad = (ΔT2 x C - Q) / m2 (2)
where ΔT2 is the temperature change that occurs during sample burning [ºC] and
m2 is the mass of the sample [g]. Hence, the gross calorific heat value of the dry
basis Qgr,d [MJ/kg] can be calculated using equation (3),
Qgr,d = Qgr,ad x 100 / (100 - Mad ) (3)
where Mad is the moisture content of the air dry sample [wt%].
The net calorific value at constant volume of biofuel and the net calorific
value at constant pressure are both obtained by calculating the gross calorific
value at constant volume as determined from the analysis sample. Calculation of
the net calorific value at constant volume requires information about the moisture
and hydrogen contents of the analysis sample. The net calorific value of the dry
basis Qnet,d [MJ/kg] can be calculated from the Qgr,d value using equation (4),
33
Qnet,d = Qgr,d - 0.02441 x M (4)
where 0.02441 [MJ/kg] is the latent heat of vaporisation of water at +25 °C and M
is the hydrogen content of the moisture-free biofuel [wt%].
The net calorific heat value as received is calculated from Qnet,d using equation
(5), where Mar is the moisture content of the as received sample
Qnet,ar = Qnet,d x (100 – Mar) / 100 – 0,02443 x Mar (5)
3.2.4 BOD measurements
Biodegradation of matter means its degradation by bacteria and micro-organisms
in soil and water environments. Various fungi may also act as decomposing
organisms in wood material and in soil. Biochemical oxygen demand (BOD) is a
measure of the quantity of oxygen required for biodegradation of organic matter
(carbonaceous demand) in water and the amount of oxygen used to oxidise
inorganic material, including ferrous iron salts and sulphides [41]. The BOD
value is an important parameter for monitoring organic pollutants in a water or
soil environment due to the simple fact that easily degradable matter will cause no
long-term risk to the environment.
EU Directive 67/548/EEC (Annex V) lists the testing requirements for new
chemicals and all chemicals declared to be on the market in September 1981
[42,43]. The accepted methods for determining biodegradation are listed in Annex
V, Part C, and the respirometric testing method is one of the accepted testing
methods for determining biodegradation. Thus, BOD OxiTop methods used for
biodegradation in a solid phase and in solution are accepted testing methods.
Furthermore, when new pellet products that include new chemicals as binding
agents are manufactured in the future, EU legislation requires that the
biodegradation of these chemicals is determined using accepted testing methods.
Biodegradation tests in solution
The biodegradation tests were carried out in this study in OECD 301F standard
conditions (optimal solution conditions) using the manometric respirometric BOD
(Biological Oxygen Demand) OxiTop method [41,44,45,46,47] and OxiTop®
Control 6 instrumentation (see Fig. 8).
34
Fig. 8. BOD OxiTop apparatus used for respirometric manometric measurements in
solution (OECD 301F conditions, ground water and surface water).
This method is based on very accurate automatic pressure measurements in closed
bottles under constant temperature (here 20.0 ± 0.2 oC). When organic matter
degrades, it requires a certain amount of oxygen, Equation (6):
Corg + O2(g) → CO2(g) (6) CO2(g) + 2NaOH(s) → Na2CO3(s) + H2O(l) (7)
When oxygen is consumed from the gas phase, the pressure falls and carbon
dioxide gas is produced, but in this method the carbon dioxide is absorbed by
solid sodium hydroxide pellets and therefore does not affect the measured
pressure, Eq. (7). The measurement time can be selected by the user (the
maximum for one measurement period is 99 days, but repeat periods can be
used), and here in the OECD 301F tests BOD28 was determined. The
measurement is fully automated, and for measurements in solutions, the
instrument calculates the BOD value in the desired unit [mg/L] using Equation
(8):
BOD[mg/L] = M(O2)/RTm x [(Vtot - V1)/V1 + αTm /T0] x Δp(O2) (8)
M(O2) is the molecular weight of oxygen (32.00 g/mol), R is a gas constant
(83.144 l hPa mol−1 K−1), Tm is the measurement temperature [K], T0 is 273.15 K,
Vtot is the bottle volume [ml], Vl is the liquid phase volume [ml], α is a Bunsen
absorption coefficient (0.03103) and Δp(O2) is the difference in partial oxygen
pressure [hPa] as given by WTW.
35
Biodegradation tests in the solid phase
Pellet samples (about 50–100 g) with a normal moisture content were first
weighed and measured in MG 1.0 bottles (WTW Weilheim, Germany) using
OxiTop® Control B6M instrumentation (see Fig. 9) [41,44,46,47]. Then, a 50-mL
beaker filled with a 1-M sodium hydroxide solution was placed on a holder. The
bottles were held in an incubation cabinet at a temperature of 20.0 ± 0.2 °C for a
period of 99 days.
Fig. 9. OxiTop® Control B6M instrumentation for BOD measurements in the solid
phase.
The BOD OxiTop apparatus for solid phase measurements is also fully
automated, but it only calculates the difference in partial oxygen pressure Δp(O2)
[hPa]. After that, consumed oxygen Δm(O2) [mg] can be calculated using
Equation (9) when free gas volume Vf is first calculated.
Δm(O2) [mg] = Δp(O2) x Vf x M(O2) / (R x Tm) (9)
The degree of biodegradation of the substance in solution and the solid phase can
be calculated using Eq. (10),
Degree of biodegradation [%] = BOD [g/g] / ThOD [g/g] x 100% (10)
where ThOD [g/g] is theoretical oxygen demand, calculated from the mass and
carbon content of the sample. The BOD value in Equation (9) is calculated using
Eq. (11):
BOD [mg/mg] = Δm(O2) [mg] / msample) [mg] (11)
36
3.2.5 Mechanical durability measurement
The mechanical durability of the pellets was tested according to the CEN/TS
15210-1 standard [48]. Sieved double samples were tumbled in standard
dimension boxes at 50 ± 2 rpm for 500 rotations and sieved. An acceptable result
is more than 97.5% of the mass above the sieve. The average of the test results
was used.
3.2.6 Microscopic structure analysis
It has been observed that the most cost-efficient binding agents in pellet
production are starch-containing materials. Therefore, various locally tailored
binding agents, such as certain industrial by-products and solid starch-containing
waste materials, are currently being considered and studied. To improve the
quality of pellets we have developed and tested a new specific optical
microscopic staining method using starch-containing binding agents.
Characterisation of starch-containing wood pellets by optical microscopic staining
has been developed as a tool for the purpose of controlling and optimising pellet
production and is described in this thesis.
The first step in the staining procedure is to mould wood pellet samples in
epoxy resin to support the pellet structure. After this step is complete, the samples
are sectioned by grinding. Every sample moulding contains two orientations of
pellets: a cross-sectional cut and an axial cut. Then the surfaces of the pellet
sample mouldings are honed, carefully polished and left to cool down for about
one hour [Paper III]. The unstained pellet samples are then digitally imaged with
a Leica MZ FL III stereomicroscope. The staining procedure is carried out with a
reagent selective to starch compounds, such as potassium iodide (KI) [49].
Colour formation caused by a chemical interaction between starch and iodine
is an important property for starch characterisation. Iodine forms a complex with
α-1.4 linked glucans by penetrating the hydrophobic cavity of the linear glucan
helices. This complex is generated if the chains are sufficiently long. In
spectroscopic determination the maximum wavelength (λmax) of the formed
complex is situated in the visible region, and the value of λmax is dependent on the
length of the chain [49].
Potassium iodide is prepared by dissolving 4–5 g of potassium or sodium
iodide in a bit of water and adding approximately 130 mg of iodine. After
dissolution of the iodine, the solution is diluted into 100 mL of distilled water
37
[50]. Potassium iodide is then carefully placed on the surface of the pellet sample
and left to absorb for a period of five minutes. After that, the staining reagents
must be carefully flushed away from the pellet samples, first with water and then
with ethanol. The samples are then dried before the second microscopic
examination. The last step of the optical microscopic staining method included
examination of both the unstained and stained pellet samples with a Leica optical
microscope and digital imaging for visual analysis purposes. The microscope
images enabled us to analyse the binding structure of the pellet and starch
penetration.
In the microscope images obtained with the method we have developed,
starch compounds appear as small dark spots and areas. Microscope images
enable us to analyse the binding structure of the pellet and starch penetration.
However, in visual analysis it must always be noted that even after careful
flushing, pellet pores may contain some reagent residuals and wood material itself
may contain dark spots, and hence these effects must be eliminated with careful
flushing in the analysis stage. There is reason to point out that these conclusions
concerning the visual analysis of the pellet samples containing starch have been
examined from the real microscopic slides, and not from the images presented in
the results. In the future, computational image analysis will be integrated into the
structure analysis procedure, which transforms the data received from staining
distribution for numerical information. Currently image analysis is based on
observing the shape of the staining distribution, for example whether binding
agent is mixed for example as lumps, uniformly distributed or distributed only in
pellet surface. As more sample points are added in the analysis, one or two quality
parameters (e.g. mechanical durability) can be attached—the mechanical
durability results of high quality pellets compared with image analysis data—and
then investigate whether regularity in staining distribution can be found. The
information obtained with this method can be used for examining and planning
the feeding of binding materials in pellet plants (place, amount, functionality) and
for optimizing the pelletizing process.
3.2.7 TG analysis
Thermogravimetric analysis (TG) is a thermoanalytic (TA) method wherein the
mass of the sample vs. temperature is weighed when the sample is heated at a
constant heating rate. With the help of TG measurements it is possible to obtain
vital quantitative information about the weight loss of the sample. TG
38
measurements also supply us with direct stoichiometric data for the purpose of
analysing certain reactions [51,52]. Hence, TG is a useful tool for developing the
drying processes of different biomasses, such as sawdust, wood chips, peat and
potato peel residue used in this thesis work. TG measurements give a simple TG
curve as a result. From TG curves it is possible to estimate the mass change of
different compounds during drying. The components under study (such as VOC,
CxHx, NOx) are determined before measurement. A curve is obtained for each
determined component so that more detailed examinations can be done later. The
information obtained can then be used e.g., to optimise the drying temperature for
different types of wet biomaterials. The TG curve of a certain potato residue
material is given as an example in Fig. 10.
Fig. 10. Thermogravimetric (TG, green line) and differential scanning calorimetric
(DSC, blue line) analysis data for potato residue material dried to a constant weight
at 105 °C.
3.2.8 Elemental analysis
The samples were extracted with MARS5 microwave wet combustion equipment
using the EPA-3051 standard method [53]. Measurements were done with a
Project:Identity:Date/Time:Laboratory:Operator:Sample:
-sample15.10.2007 13:52:29ProcessmetallurgyRMA-, 49.800 mg
Material:Correction File:Temp.Cal./Sens. Files:Range:Sample Car./TC:Mode/Type of Meas.:
base-12102007.bsvlpt-dsccp-210607.tsv / sens-dsccp-210607.esv30/20.0(K/min)/1000DSC(/TG) HIGH RG 2 / SDSC-TG / Sample + Correction
Segments:Crucible:Atmosphere:TG Corr./M.Range:DSC Corr./M.Range:
1/1DSC/TG pan PtAir/50 / ---/--- / ---/---020/30000 mg020/5000 µV
Instrument: NETZSCH STA 409 PC/PG File: F:\Muut\Raudaskoski\sample-15102007.dsvMarko 2007-10-15 15:47 Main
100 200 300 400 500 600 700 800 900Temperature /°C
-5
0
5
10
15
20
25
30
35
DSC /(mW/mg)
10
20
30
40
50
60
70
80
90
100
TG /%
[1][1]
↑ ex
39
commercial plasma emission spectrometer (ICP-OES) and the results were
calculated against the sample’s dry weight (at 105 °C).
3.2.9 Particle size distribution
Particle size was determined with a Beckman Coulter LS 13 320 laser diffraction
particle size analyser. The range of measurement was 0.4 to 2000 µm. The
particle size analyser utilises laser diffraction as a measurement method. The laser
diffraction technique is based on the principle that a laser beam scatters light at an
angle that is directly related to particle size. The scattering angle increases
logarithmically when particle size decreases. Small particles scatter light at wider
angles with low intensity and large particles scatter light at narrow angles with
high intensity [54].
3.2.10 Sequential leaching procedure
As presented in the introduction and later in the results section, in EU countries
both utilisation and disposal of bio-ashes and other waste materials require
chemical analyses of the materials using different analytic methods. These
standard methods include determination of either total concentration or that of
dissolution in water (either a two-stage leaching test with L/S ratios 2 and 8,
cumulatively 10:1, or an up-flow percolation test, CEN/TS 14405:2004 [55]) for
different species, thus providing information on dissolution in only one selected
condition. The most important environmental legislation concerning bio-ash and
other waste materials in Finland today are: i) Waste Act 646/2011 [18], ii)
Ministry of Agriculture and Forestry Decree on Fertilizer Products 24/2011 [56],
iii) Government Decree VNa 591/2006 [57], or Utilisation of Certain Waste
Materials in Earth Construction Government Decree VNa 214/2007 [58], iv)
Assessment of Soil Contamination and Remediation Needs (“the Finnish PIMA
decree”) VNa 214/2007 [58] and v) Government Decree on Amendment of
Government Decision on Landfill Sites VNa 202/2006 [59]. Finnish
environmental legislation is in accordance with the corresponding EU legislation
[60,61], as well as in agreement with the principles of sustainable development,
LCA (life cycle assessment) and material efficiency. Finland also has
environmental legislation on some issues related to wastes that have not yet been
covered by EU legislation. Lastly, it is worth mentioning that in these
40
requirements, waste-related legislation in Finland has undergone an extensive
reformation during the last few years.
The single chemical determination methods used in EU and Finnish
environmental legislation and mentioned above give only poor information for
risk assessment of different species and tell nothing about the effect of the
environment on the solubility, mobility and bioavailability of harmful heavy
metals. This information is necessary if we wish to know the real environmental
risk of metals in different conditions possible in nature now or in the future, and if
we wish to substantially increase utilisation of waste materials according to EU
and national strategies and in tandem reform environmental legislation.
Therefore, during the last couple of decades many different sequential leaching
procedures have been developed—most of which are based on the method
initially developed for sediments by Tessier et al. [62]—and used to determine
trace elements and sulphur in different materials. During the last decade, a
research group at the University of Oulu has studied and published many papers
on the use of a sequential 3–5-stage procedure for estimating the bioavailability,
mobility and thus risk assessment of many different, especially bio-based, Finnish
waste materials and industrial by-products, such as fly or bottom ash from
industrial and municipal district heating plants [Paper V,63,64], bottom or fly ash,
paper mill sludge, green liquor dregs, PCC waste and lime waste from pulp and
paper mills [65,66,67,68,69,70,71] and waste rock material originating from a
Finnish zinc mine and used as railway ballast in Northern Finland [72,73].
The five-stage sequential leaching procedure (see Fig. 11) generally used to
determine the distribution of 11 metals (Cd, Cu, Pb, Cr, Mo, Zn, As, Co, V, Ni,
Ba), and sulphur (S) in the studied materials into the following fractions: 1) a
water-soluble fraction (ultrapure distilled water acidified with HNO3, pH = 4.0),
2) an exchangeable fraction (CH3COOH), 3) an easily reduced fraction (NH2OH-
HCl), 4) an oxidizable fraction (H2O2 + CH3COONH4) and a residual fraction
(extraction with HF + HNO3 + HCl). The conditions in the last step—three strong
acids and strong, long-lasting shaking—are never possible in nature, and therefore
the residual fraction is called the inert phase.
Since the conditions in stages (1)–(4) represent realistic possible risk
conditions at least in the future, the potential bioavailability percent PBPM [%]
can be calculated from the sum of the concentrations of M in stages (1)–(4), or
(2)–(4) if the water extraction stage (1) is missing (as in paper V, and the total
concentration ctot(M), by using equation (12), where ci(M) is the concentration of
metal M in stage i [Paper V, 69,70]. The value of ctot(M) can be calculated either
41
from the sum of the concentrations in all stages or directly by some standard
determination method. The values of residual percentage and concentration can be
calculated with equations (13) and (14).
( )
( )
4
1 100%ii
Mtot
c MPBP
c M== ×
(12)
[ ]% 100% Mresidual PBP= − (13)
( )( ) 100residual M totc M PBP c= − × (14)
Fig. 11. Five-stage sequential leaching procedure [Paper V].
42
43
4 Pelletizing machinery
A pellet factory usually includes certain unit processes relative to pelletizing
(pelletizing machine, cooler, conveyors, recycling system, mixers). However, a
drying unit has only recently been in use in a few factories. Sawdust may be
brought to a pellet factory already dried, which increases the cost of the raw
material, or it may be pelletized without discrete drying, which lowers the quality
of the pellet product. The main unit processes of a pellet factory are described in
the following chapters.
4.1 Delivery of raw material and receiving stock
The location of a pellet factory is ideal if there is a sawmill, for example, located
nearby. In that case the source of raw material is close and the operation is
logistically cost-effective. The by-products of a sawmill (sawdust and cutter
shavings) can be utilised as the raw material of pellet manufacture, and thus
closely located factories create a symbiosis that is ecologically beneficial to both
sides. Furthermore, if the pellet plant has a drying unit, the drying process can be
performed by this unit and the pellet factory can receive moist raw material.
As the first step in a pellet factory the undried raw material is transported by
a wheel loader into receiving silos in the factory where the raw material is
preheated by recycling waste heat from the later stages of the process. This is
necessary especially in winter, because the raw material may be covered with
snow and heating it before it is transferred to a drying unit saves energy in the
drying phase and minimises the blockages in the later process stages. The
preheated biomass is fed forward from the receiving silos by a pneumatic blower.
4.2 Drying unit and pre-treatment of raw material
A pneumatic blower moves sawdust from the receiving silos to a conveyor which
transports the preheated to a drying unit. Wet sawdust is fed into the feeding tube
of a drum drier by a screw conveyor. In the drying unit moisture is removed from
the raw material and the generated waste heat is used to heat the receiving silos.
The typical parameters of a drum drier (diameter, length, permeability, drying
time, drying temperature) depend on the moisture and quality of the raw material
being dried. Drying is performed as a continuous process. The drying temperature
44
of a drum drier can be between 100 °C and 380 °C. Drying is carried out until the
required moisture content in the pelletizing process (less than 15%) is achieved.
After the drying process and before the pelletizing machine, the first
separation of dust is done by a cyclone. The separated dust is salvaged by the
cyclone and recycled back to the receiving silos of the process. Multiphase
separation of dust is necessary for a good-quality pellet product, and drifting of
dust among the pellet product must be avoided. Dust causes significant problems
in the combustion process, such as explosions and a decrease in combustion
efficiency.
Before being fed to the pelletizing machine, the dried raw material is
collected into an intermediate mixing chamber. The raw material is crushed by a
crusher in to make the material homogenous enough to be fed into the pelletizing
machine. This is done to avoid blockages in the pelletizing machine. Blockages
cause both interruptions in production and abrasion of the machinery. The
intermediate mixing chamber is meant especially to prevent interruptions in
production. To keep the raw material from getting lumpy, the intermediate mixing
chamber contains a discrete mixer.
4.3 Pelletizing machine
The dried raw material is fed by conveyor to a pelletizing machine. The
pelletizing machine is the most essential stage of the pelletizing process in terms
of the quality of the pellet product. The pelletizing machine feeds the raw material
into a pellet press, possibly adds binding agents and presses the pellets.
Pelletizing is a mechanical pressure operation. The high pressure and friction of
the press causes the temperature of the wood to increase significantly, thus
slightly plasticising the lignin. This phenomenon is very important, especially in
forest biomasses with large lignin contents. Hardwood contains ca. 20% lignin
and softwood even more, ca. 30% [74]. As a rule of thumb, a pelletizer requires
between 50 and 100 kW of electrical demand for every ton per hour of production
capacity. In addition, electricity is usually needed to operate any chopping,
grinding, drying, cooling, and bagging equipment that is in use. Electrical demand
is dependent on many factors such as raw material, matrix channel and binding
agent properties.
Two different types of pelletizing machines are commonly used in the pellet
industry: a plane matrix machine (Fig. 12) and a core matrix machine (Fig. 13).
45
Fig. 12. Plane matrix pelletizing machine [75].
In a plane matrix machine pressing occurs through a round planar matrix where a
rolling press rolls against its surface (Fig. 12). The pressing mechanism is based
on either a rotating matrix and stationary rolls, or a stationary matrix and a
rotating roller head, where the rolls rotate axially around a main shaft. In a plane
matrix machine feeding is done by force of gravity from above. The most
important advantage compared with a core matrix machine (see Fig. 13) is that in
this case the matrix is easy to clean.
In the second and most common type of pelletizing machine pressing occurs
through a core matrix (Fig. 13). The pellet pressing mechanism is based on a
stationary core matrix and revolving press rolls inside the inner circle or a rotating
core matrix with stationary axially rotating press rolls. In a core matrix pelletizing
machine the amount of raw material fed and the binding mechanism are the two
most decisive factors in achieving high efficiency as well as low, uniform wear of
the matrix.
46
Fig. 13. Core matrix pelletizing machine [75].
47
4.4 Pelletizing field tests
It is generally known that the external conditions in a pellet plant (temperature,
atmospheric moisture, etc.) can vary strongly between different seasons and even
during the same day, which can strongly affect the success of wood pelletizing.
However, it is necessary to also obtain results from pelletizing in large-scale field
conditions in addition to pilot-scale pelletizing experiments at constant
conditions. Therefore, the samples used in this thesis work were manufactured by
different pellet facilities, from pilot-scale to industrial-scale processes.
Fig. 14. Process description of the Formados Inc pellet plant in Kuusamo [76].
In our field tests we used three different pellet plants to manufacture the pellet
samples. In the Formados Inc. pellet plant in Kuusamo, binding materials were
added to a continuous flow of raw material just before the pressing phase in the
pelletizing machine (Fig. 14). The core matrix compressed pellet samples were
then collected into sample trays for analysis. To calculate the amount of binding
material added, the pellet plant’s raw material input was estimated as being
48
constant during the addition periods, and the output per hour was estimated from
the pellet plant’s production rate. The production capacity in the field tests of the
Formados pellet plant was about 1500 kg/h.
In the Mekrijärvi Research Station experiments (Fig. 15), additives are added
to a continuous flow of raw material on a collector screw before the mixer buffer
silo. Pellet samples were collected in sample trays for analysis after the cooling
phase. To calculate the amount of binding material added, the pellet plant’s raw
material input was estimated as being constant during the addition periods, and
the output per hour was estimated from the pellet plant’s production rate. The rate
of addition of binding material was calculated based on calibration to correspond
to the required binding material contents (1% and 2%). The production capacity
in the tests of the pilot pellet plant in the Mekrijärvi Research Station was about
300 kg/h.
Fig. 15. Pellet facility used in this study. 1) Drier carriage and container, separate
locations, 2) Rough sieve (30-mm square holes) + metal trap, 3) Rough sieve bypass
flow, 4) Hammer mill (Miller 20), 5) Spiral conveyor from hammer mill to raw material
silos, 6a-c) Raw material silos 1–3 (3.5 m3 each), 7a) Collector screw, 7b) Collector
screw operation at raw material silo calibration, 8a&b) Additive addition 9) Buffer
mixer silo, 10) Pellet press feed screw conveyor 11) Pellet press, 12) Belt conveyor to
cooler 13) Cooler 14) Vibration sieve conveyor (5-mm round holes). Automation, dust
extraction and blowers are not shown [Paper II].
In the EkoPelletti R&D project’s pilot pellet plant at Oulu University of Applied
Sciences, School of Renewable Natural Resources, binding materials were added
to a continuous flow of raw material just before the pressing phase in the
pelletizing machine (Fig. 16). Core matrix compressed pellet samples were then
collected into sample trays for analysis. To calculate the amount of binding
49
material added, the pellet plant’s raw material input was estimated as being
constant during the addition periods, and the output per hour was estimated from
the pellet plant’s production rate. The production capacity in the tests of the pilot
pellet plant at Oulu University of Applied Sciences was about 150 kg/h.
Fig. 16. Pilot pellet plant at Oulu University of Applied Sciences, School of Renewable
Natural Resources.
50
51
5 Experimental work
The research of this thesis work was carried out mainly at the University of Oulu
in close co-operation with Finnish pellet factories, forestry experts and Finnish
industry. In the preliminary stage, full industrial-scale manufacturing of our
sample pellets was carried out at the Formados Inc. pellet plant (see Fig. 14) in
Kuusamo, in North-East Finland (Paper I). In the preliminary tests it was
observed that external conditions (temperature, atmospheric moisture, etc.) can
vary strongly between different experiments, and therefore later samples were
manufactured in pilot-scale pellet facilities at constant conditions. The samples of
pellets manufactured in these preliminary tests were used also in characterisation
of wood and wood-based bio-ash (Paper IV and V).
Pilot-scale manufacturing of our sample pellets for paper II was carried out at
the Mekrijärvi Research Station pellet facility in Ilomantsi (see Fig. 15). Some
supplementary tests and pellet manufacturing were carried out at the pilot pellet
plant in Oulu University of Applied Sciences, School of Renewable Natural
Resources (see Fig. 16). The pellet samples for staining method analysis for paper
III were manufactured from softwood from Central Finland at a typical pellet
plant by Vapo Inc., which is the largest pellet producing company in Finland.
The moisture content and BOD measurements (Paper I and II) were carried
out at the Department of Chemistry. The calorimetric heat measurements (Paper I
and II) were carried out at the Department of Chemistry as well as in
Finnish Forest Research Institute Metla in Kannus. Staining method analysis
(Paper III) was carried out in co-operation with Vapo Inc. at the Department of
Chemistry and at the Fibre and Particle Engineering Laboratory at the University
of Oulu. Particle size distribution measurements (Paper II) were done at the Fibre
and Particle Engineering Laboratory. TG analysis measurements were carried out
at the Laboratory of Process Metallurgy at the University of Oulu. Sequential
leaching procedure and solubility tests of bio-ashes (Papers IV and V) were
carried out at Suomen Ympäristöpalvelu Oy, a FINAS-accredited test laboratory.
52
53
6 Results and discussion
This chapter presents the chemical results of our studies on the development of
eco- and material-efficient pellet production. The results are collected from
analyses in pre-experimental pelletizing research, the publications (I-V) of this
thesis and in addition some new results from the EAKR EkoPelletti R&D project.
Some pre-experimental tests have been carried out just recently. There is reason to
point out that in addition to these practical pelletizing results, we have developed
and tested new chemical methods for use in pellet research, in addition to the
methods used in current pellet standards. An optical staining procedure and an
automatic respirometric BOD OxiTop method are the most important of these
new chemical methods used for pelletizing studies.
6.1 Moisture content
Drying of biomass has increased during the last decades and with a growing pellet
market it will increase further. The fresh wet raw material of wood pellet
production contains about 50–55% water. For pellet production, manufacturers
normally dry the raw material to a water content of 8–12% before the pelletizing
process begins [77]. Drying of wood-based biofuels is important, since wet wood
results in low combustion temperatures, low energy efficiency and high emissions
of hydrocarbons and particles compared with, for example, pellets. If biofuels are
dried and compressed into pellets or briquettes, the fuel will have controlled
moisture content (MC) and a higher energy density and it is easier to transport. It
will also take up less room and is also less biodegradable and less susceptible to
mould and insect attacks during storage.
Samuelsson et al. found that the moisture content of sawdust was the
dominant factor of bulk density and the pelletizer motor current; they both show
low values at high moisture contents due to the lubricating property of water,
which lowers friction in the pelletizing process. The increase in binding strength
with longer storage time is explained by a reduction in extractive content which
contains molecules that can block binding sites on the material surface. Optimum
pellet quality was obtained when storage time exceeded 120 days and with
sawdust moisture content within the range of 11–13% [78].
In this thesis work the raw materials of the pellets manufactured were already
dry. We modelled the drying process of different biomasses (sawdust, wood chips,
peat) used in pellet manufacture. The results showed up as asymptotic curves for
54
each raw material (see Fig. 17). For peat the curve was almost linear. Wet sawdust
(moisture content 51.5%) needed to be dried 60 minutes to achieve the required
pelletizing dryness. Fresh wood chips (moisture content 54.4%) needed to be
dried up to 150 minutes to achieve the required pelletizing dryness. For the peat
the required pelletizing moisture content was achieved in 300 minutes. Drying
temperature in the test was 105 °C.
Fig. 17. Drying curves of pellet raw materials at drying temperature T = 105 °C.
6.2 Calorimetric heat (I,II)
All the experiments showed that lignosulphonate, potato flour/waste potato flour
or potato peel residue did not have any significant effect on the calorimetric heat
value of wood-based pellets. This is logical, due to the low content of the binding
material in the pellets and the differences between the heat values of the analysed
wood and binding materials. Calorimetric heat results in this study are presented
as gross calorific heat (Eq. (2)), net calorific heat (Eq. (3)) and net calorific value
in received moisture (Eq. (4)). The heat values of wood pellets containing bark-
free stem wood and different binding material are presented in Fig.18. Gross
calorific heat values amongst the different pellet samples are not proportional to
changes in moisture content between the samples. Therefore, the results presented
55
as gross caloric heat values are usually the most representative amongst all the
samples.
The results show that additives generally comprise less than 2% of total mass,
and therefore they have only little effect on the combustion of pellets. Even if the
maximum amount of a high-energy-content additive, like vegetable oil, were
used, the total effect would be a less than 3% increase in heating value [79].
Fig. 18. Heat values [kJ/g] of wood-based pellets containing stem wood and binding
material.
However, owing to the accuracy of the combustion heat values observed—for
example lignosulphonate-containing pellets with an increasing quantity of
lignosulphonate—the results can be explained by the slightly lower gross calorific
value of lignosulphonate compared with the corresponding value of the studied
stem wood: Qgr,d (stem wood) = 20.31 kJ/g > Qgr,d (lignosulphonate 1%) =
20.21 kJ/g > Qgr,d (lignosulphonate 2%) = 20.12 kJ/g > Qgr,d (lignosulphonate)
= 17.06 kJ/g.
All the experiments show that measurement of the calorific heat value with
an automatic calorimeter is very accurate. However, there is an interesting
question: Is there uniformity in the calorimetric heat values determined by
different calorimeters and their users? Therefore, we carried out an inter-
56
calibration study to examine this problem. This is especially important in order to
be able to compare different heat value results determined in different
laboratories. The results of the inter-calibration study are presented in Table 2
[unpublished results].
Table 2. Qgr,ad results of an inter-calibration study with different calorimeters [MJ/kg].
Sample IKA C200
purchased 2011
IKA C2000 Basic
purchased 2008
IKA C5000
purchased 2005
Parr 6200
purchased 2010
Oat straw pellet 19.02 18.99 18.88 18.79
Sawdust pellet 20.67 20.87 20.83 20.81
Chips/straw/oat pellet 19.80 19.86 19.83 19.68
Biodiesel 39.82 39.68 39.9 39.84
Chain oil 39.50 39.32 39.5 39.46
Butanol 35.97 35.63 35.9 35.77
The results of the inter-calibration study [unpublished data] showed that:
1. The heat values Qgr,ad of the studied liquid biofuel samples are significantly
higher than the heat values of the solid samples.
2. The liquid samples are homogenous, and so the heat values of different
laboratories are very uniform, dispersion between the different liquid samples
being only ± 0.1, ± 0.1 and ± 0.2 [MJ/kg].
3. Considering the practical point of view (biomasses are not totally
homogenous), the solid samples were not strictly homogenised, but still
dispersion between the results was remarkably small (all three ± 0.1
[MJ/kg]), being of the same magnitude as that of the homogenous liquid
biofuels.
4. Because of their inhomogeneity, it is not reasonable to express the results of
the natural products such as solid biofuels more than with only one decimal
of accuracy. In addition, the sampling error of solid material [80,81] also
affect to determined heat value and must be taken into account in result
accuracy.
5. The brand, model (price), age nor performer of the determination have no
significant impact on the heat value results, as long as the researcher does all
the determinations carefully.
6. The results showed that all the heat values of the four different laboratories,
determined by bomb calorimeter for the liquid and solid biofuels, are
57
remarkably uniform, which knowledge can be utilised in collaborative studies
in the future.
7. These Qgr,ad results can be used as an approximation of biofuel heat values of
liquid (in Table 2: 38 ± 2 MJ/kg) and solid biofuel samples (in Table 2: 20 ± 1
MJ/kg).
6.3 BOD measurements (I,II)
Biodegradation of pellets and their raw materials is a very important factor in full-
scale industrial manufacture of pellets, as high biodegradability may cause
considerable economic losses during storage and transportation. Potato peel
residue is moderately biodegradable when measured in OECD 301F conditions.
Its degree of biodegradation is 27% in 5 days (Fig. 19). Even though potato peel
residue itself has high biodegradability, it seems to even decrease the
biodegradation of pellets when it is used as a binding material. This effect can be
explained by the prevention of oxygen gas adsorption in pellet pores. The effect
of lignosulphonate is slight but opposite, it increases the biodegradation of pellets.
Lignosulphonate is not biodegradable when measured in OECD 301F conditions.
Its degree of biodegradation is 1.75% in 7 days of BOD7 measurement. Potato
flour is slightly biodegradable when measured in OECD 301F conditions. Its
degree of biodegradation is 4% in 7 days of BOD7 measurement. The graph of
potato peel residue turns downward after 5 days. This may be explained by
moulding or some other reactions in the potato peel residue, which causes gas
formation (also observed as malodorous gases). This causes an increase in
pressure during the period from 5 to 16 days, after which biodegradation or other
reactions that form carbon dioxide are dominant.
58
Fig. 19. Biodegradation of binding agents in solution in OECD 301F test conditions
within 28 days.
Fig. 20. Biodegradation of pellets with no binding agent and lignosulphonate pellets in
the solid phase within 30 days.
When biodegradation was measured in solid-phase pellets without a binding agent
and in pellets containing lignosulphonate as a binding agent, they did not degrade
at all (Fig. 20). Fresh wood chips with a moisture content of 50% were used as a
59
reference material. Pellets containing potato peel residue and potato flour
measured in the solid phase did not degrade at all, as can be seen in Fig. 21. The
biodegrability results of this study show that BOD reactions in the analysed
wood-based pellets do not have any practical effect on their mechanical
durability, nor do they cause mass loss even during a relatively long period of
storage. However, it is generally known that different fungi and other micro-
organisms decompose moist wood in the solid phase.
Fig. 21. Biodegradation of potato peel residue pellets and potato flour pellets in the
solid phase within 30 days.
The biodegrability of condensate water from drying of mixed wood chips was
also determined for some pre-experimental cases (Fig. 22) [unpublished results].
This knowledge is very important for future consideration of condensate water
from drying of mixed wood chips on an industrial scale. Also pH, TOC, COD and
electrical conductivity were determined. GC-MS results showed that the
condensate water in question did not contain any PAH compounds. The degree of
biodegradation (BOD28) of organic matter in condensate water was 41% (average
of 3 determinations). The BOD28 value shows that condensate water from drying
of mixed wood chips is moderately biodegradable and purification of condensate
water from wood chips is spontaneous. All these results—which indicated a lack
of heavy metals in these waste waters—suggest that at least the analysed
condensate water from drying of Finnish wood chips is pure and its treatment
causes neither problems nor costs [unpublished results].
60
Fig. 22. Biodegradation of condensate water from drying of mixed wood chips (pine
20% and aspen 80%) determined by the BOD OxiTop method at 20 oC.
6.4 Binding agent usage (I, II)
Several explanations have been proposed for the mechanisms by which particles
in binderless wood composites, including wood pellets [82], are bonded, such as
thermal softening of lignin [83], pectin bonding [84], bonding through
hemicellulose degradation products [85] or mechanical particle interlocking [86]
in which the size, shape and surface roughness of the wood particles influence
bonding strength [87]. In this thesis work we concentrated on studying the effects
of using a binding agent to improve pellet compactness and quality.
The influence of raw material extractives and binding agents on pellet
characteristics such as density, compression strength and moisture sorption is not
well understood. However, it has been observed that active sites on the particles
may be blocked by lipophilic substances that have migrated to the particle surface
during drying and hot pressing, thus obstructing wettability and reducing bond
strength [87,88]. It may be assumed that similar hydrophobic extractive migration
to the surface also occurs during sawdust drying and pelletizing, thus affecting
particle bonding in the pellet [89]. This view is supported by the fact that pellets
pressed from fresh sawdust compared with those pressed from stored sawdust
-5
0
5
10
15
20
25
30
35
40
45
0 5 10 15 20 25 30
BOD/ThOD*100%
time [d]
61
show differences in durability. Pellets pressed from stored sawdust or from a
mixture of fresh and stored sawdust generally have higher durability and bulk
density than pellets pressed from entirely fresh sawdust [90,91]. During the
storage of sawdust, VOC compounds may evaporate and lipophilic extractives are
broken down through microbiological and auto-oxidative processes [91,92,93],
resulting in increased particle surface wettability and improved particle bonding
[87,88]. Stronger bonding between particles will most likely minimise springback
after compression, thus resulting in higher density [89]. Knowledge about the
influence of extractives and binding agents on the pellet’s physical characteristics
could make it possible to fine-tune the raw material handling process in the
pelletizing industry, resulting in less variation in pellet quality.
Additives or binding agents may be added to improve the pellet’s mechanical
properties, i.e. to increase strength and density, improve pelletizing throughput or
improve moisture resistance [94]. Another reason to include additives is to
improve combustion properties, e.g., the ash melting point, slagging and
corrosion [95]. Binding agents that improve inter-particle bonding may be in
liquid or solid form. It is reported that roughly 50 organic and inorganic
compounds have been used in densified biomass products [96]. In feedstock
pellets, a wide range of additives are used, including different starch-containing
compounds, molasses, proteins, modified cellulose, lignosulphonate [Paper II],
kraft lignins and inorganic clay minerals [6,97,98]. Other additives used in wood
pellets are paraffin, stearin and cellulose fibres [94]. It has to be noted that there
are national differences on what additives and binding agents are allowed in fuel
pellets. However, from the environmental, economic and logistical points of view,
the most important binding materials are industrial by-products and locally
utilisable residuals.
The aim in the future is to expand the base of wood pellet raw material by
utilising more low-value and/or moist biomass, especially logging residue from
thinning. To improve the quality of pellets, commercial and industrial by-product
materials are commonly used as binding agents in pellet production [4,6,12].
Also, considering the profitability of production and some occupational safety
problems (wood dust exposure, fire and explosion risk, coherence, etc.), it is
practical to use binding agents. By using some new binding materials, such as
potato peel residue, it seems to be possible to prevent pellet biodegradation [Paper
I], which can cause substantial economic losses in full-scale pellet production.
The aim of the present research was to find and test some eco- and cost-efficient
materials for binding purposes and thus improve the competitiveness of pellet
62
products as an alternative in energy production. Therefore, various locally tailored
binding agents such as certain industrial by-products and residuals, especially
starch-containing waste materials, are currently under study. In this thesis
lignosulphonate, residual potato flour and potato peel residue were used and
tested as adhesive binding agents. Using lignosulphonate, potato flour and potato
peel residue as model agents in pelletizing, in this thesis we tested the total
functionality of a pilot-scale pellet facility combined with an extensive chemical
toolbox (see Table 1) to promote future development of eco- and cost-efficient
wood-based pellet production in both the quantitative and qualitative sense.
The power consumption of the press was measured, and its efficiency can be
estimated by comparing its power consumption to the feed rate of material to the
pellet press. Sample pellets were produced with the settings described in Table 3.
Potato peel residue was problematic in terms of moisture. The 5% potato peel
residue mixture was excessively dry, which increased power consumption. To
produce good-quality pellets from a 20% potato peel residue mixture, a slower
feed rate was required to let moisture evaporate from the pellets during the
pressing process. Lignosulphonate has the best properties as an additive; while
improving pellet quality, power consumption does not significantly increase
compared with native wood. Adding potato flour does increase power
consumption, but it also improves pellet quality while having no noticeable effect
on inorganic content.
Table 3. Efficiency: power consumption, feed rate to the pressing chamber and pellet
outputs.
Power consumption (kW) Feed Power/feed Pellet output (kg/h)
Native wood (no additive) 21.8 7 3.1 144
Potato flour 1% 21.7 6 3.6 a
Potato flour 2% 23.5 5 4.7 a
Potato peel residue 5% 21.4 5 4.3 116
Potato peel residue 10% 19.0 5 3.8 a
Potato peel residue 20% 17.6 3 5.9 a
Lignosulphonate 1% 22.2 7 3.2 180
Lignosulphonate 2% 23.5 7 3.4 180
a Not determined
63
In terms of mechanical durability, native wood pellets without additives and 5%
potato peel residues (0.5 % starch) have substandard quality [48] (97.5% above
sieve). It is possible to achieve standard-quality pellets from Scotch pine without
additives, but it requires optimisation of the matrix channel length. In this thesis
work the matrix channel length was kept constant to preserve the comparability of
the results. A major factor in the case of 5% potato peel residue pellets is the
excessive dryness of the pelletized mixture, which also caused reduced output.
Fig. 23. Results of the pellets’ mechanical durability [Paper II].
All three tested binding materials had a positive effect on pellet compactness.
Lignosulphonate is known to improve pellet quality as well as compactness
[99,100]. The results of the pellets’ mechanical durability are presented in Fig. 23.
The results show that the technically optimal proportion of the analysed binding
compounds in the pellets is ca. 1 to 2%. These results are in accordance with
other experiments in both pilot and full-scale pellet plants [Paper I]. The lowest
64
effective limit for each additive needs further research in regard to elemental
composition and to obtain optimal economy. As mentioned, all the additives had a
positive impact on the pellets’ mechanical durability. In addition, lignosulphonate
seems to have a lubricating effect, which reduced the power consumption of the
pellet press during the experiments. Lignosulphonate raised the production rate of
pellets, bringing energy and cost savings, but the amount must be ≤ 0.5% in order
to keep the sulphur concentration of the pellets below the normative limit of
CEN/TS 14961 [5].
As seen in the results of paper II, potato peel residue, as an agricultural
biomass, typically has a very high potassium content [101]. According to a new
Decree on Fertilizer Products [55], the sum ratio of potassium (K) and
phosphorus (P) combined should be at least 2%, and this high potassium content
in potato peel residue can be utilised in ash products in the future. Potato peel
residue is an attractive additive from the economic and environmental points of
view, despite problems in logistics. The current cost situation related to potato
peel residue in Finland is sensitive to freight costs, whether the buyer or seller
pays for it. Potato peel residue itself has zero value and it mostly goes to cattle
feed or bioethanol production. If potato peel residue could be used as a pellet
additive, then the potato operator could reduce the need for waste management
and would have the option of selling this by-product. This requires good logistics
or local solutions, as the functional component in potato peel residue is starch,
which is quickly lost due to biodegradation and other chemical reactions. Also the
risk of earth contamination has to be taken into account, as potato peel residue is
not a purified product [28]. Potato flour is a much more refined product than
potato peel residue, but it is a standard additive in current pellet production [6].
6.5 Visual analysis of pellet sample images (I, II, III)
In the microscope images based on the method we have developed, starch
compounds appear as small dark spots and areas. Microscope images enable us to
analyse the binding structure of the pellet and starch penetration. However, in
visual analysis it must always be observed that even after careful flushing, pellet
pores may contain some reagent residuals and wood material itself may contain
dark spots, and hence these effects must be eliminated in the analysis stage. Some
images of starch-containing pellets obtained with the optical microscopic staining
method are presented in the following chapters. In the future, computational
image analysis will be integrated into the analysis procedure. In order to examine
65
microscopy photographs in more detail, specific starch penetration analysis
technologies have been developed that provide reliable information on
penetration [102]. Once more, there is reason to point out that these conclusions
concerning visual analysis of the pellet samples containing starch are based on
examinations of real microscopic slides, and not of the images presented in the
results in the following chapters.
The reference sample (see Fig. 24) contains no binding agent. As there are no
dark spots or areas to be observed in the images, this observation combined with
later results for starch-containing pellet samples show that our characterisation
method is proven to function selectively for starch compounds.
Fig. 24. Reference samples unstained (left) and stained with KI (right).
Solid starch waste pellets with binding agent contents of 1 % and 3 %,
respectively, are presented in Figs. 25 and 26. Starch compounds appear in the
images as small dark spots and areas. It can be observed that starch compound
penetration is relatively uniformly distributed in both samples. It can also be
observed that the pellet samples with a binding agent content of 3% have
distinctively more dark spots compared with a binding agent content of 1%. Thus,
our characterisation method appears to function quantitatively correct in terms of
binding agent content examination.
66
Fig. 25. Solid starch waste pellets with 1% binding material content; unstained (left)
and stained with KI (right).
Fig. 26. Solid starch waste pellets with 3% binding material content; unstained (left)
and stained with KI (right).
Liquid starch waste pellets are presented in Fig. 27. Starch compounds appear in
the images also as small dark spots. Starch compound penetration in the liquid
waste pellet samples is observed to have a more varied distribution compared
with the solid starch waste pellet samples (see Figs. 25 and 26).
67
Fig. 27. Liquid starch waste pellets; unstained (left) and stained with KI (right).
Lignosulphonate pellets and solid starch-containing potato waste pellets were
both manufactured in two different process conditions. The aim was to
manufacture pellets of both high and poor quality. After that, both samples were
visually analysed and compared. In order to obtain information on their quality
parameters and mechanical stability, the pellet samples were vibrated, sieved and
weighed. The proportions were calculated from the weighed mass in each sieve.
The conclusion can be drawn from the results of the sieve tests that the more the
cumulative proportion of the particle size exceeds 4 mm, the better the quality and
compactness of the pellet will be. In comparison to pellets without binding agents,
the effect of the different binding agents on the mechanical stability of the pellets
was concluded based on the sieving results. The moisture contents of the pellet
samples were also measured, because this is known to have an effect on pellet
quality. In our preliminary research, we observed that the poor quality of pellets is
clearly visible from optical microscopic images for example as transparency and
irregularity of shape. Some of these results are presented in Table 4. The effect on
pellet quality (transparency and irregularity of shape) was particularly observed
(see Figs. 28 & 29). These effects can be observed much more clearly from real
size microscopic slides.
Table 4. Moisture content and sieving results of two different process conditions; high
quality (sample 1) and poor quality (sample 2) pellets.
Binding agent Measurement Sample 1 Sample 2
Lignosulphonate Moisture content 6.5% 9.5%
Sieving (> 4 mm) 99.7% 96.9%
Solid potato waste Moisture content 8.0% 10.0%
Sieving (> 4 mm) 99.6% 95.2%
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Fig. 28. Lignosulphonate pellets; high quality (left) and poor quality (right).
Fig. 29. Solid potato waste pellets; high quality (left) and poor quality (right).
6.6 TG analysis
Thermogravimetric (TG) measurements results provide vital quantitative
information about weight loss in the sample, wherein the mass of the sample vs.
temperature is weighed as the sample is heated at a constant heating rate. From
the TG curves it is possible to estimate the change in mass of different gaseous
compounds during drying. The information obtained can then be used e.g., to
optimise the drying temperature of different types of wet biomaterials.
To model the drying process and optimise the drying temperature, TG
measurements were carried out for two different wood pellet raw materials: wet
sawdust and wood chips. Before thermogravimetric measurements the wood
pellet raw materials were artificially moisturised to about a 40% moisture content,
prepared for thermoscale sample (particle size 6 mm) and placed in the corundum
69
(Al2O3) beaker. Measurements were carried out under air (20% O2 / 80% N2)
atmosphere and sample heated at rate of 10 °C/min, from 25 °C to a final
temperature of 400 °C.
Fig. 30. Thermogravimetric mass curve (wt-% vs. temperature) of wet sawdust raw
material.
The thermogravimetric mass curve of sawdust raw material is presented in Fig.
30. From the graph it can be observed that elevating the drying temperature first
extracts only water (moisture) from the sawdust material, which is indicated as an
initial weight decrease in the sawdust material mass curve. This decrease in the
mass of the wet sawdust material takes place until about a 100 °C drying
temperature in the conditions of this TG study. The next sharp weight decrease in
the mass curve indicates the drying temperature which must not be exceeded
during the drying process, because in that case the sawdust material will catch
fire. For the wet sawdust material this temperature is achieved at about 230 °C.
Based on the thermogravimetric mass curve results, the maximum drying
temperature of the wet sawdust material in this study is about 200 °C. On the
other hand, it is observed from the graph (Fig, 30) that the optimum drying
temperature for wet sawdust material is about 100 °C, where most of the moisture
is evaporated but the sawdust raw material will not yet catch fire and volatile
70
compounds with a high heat value are not lost. Our results were similar with
earlier TG study of biomasses, for example Darvell et al. [103] have indicated
that the first mass loss at about 120 °C and below is from moisture evaporation,
followed by a larger mass loss due to the volatile matter being released. The
remaining residue is char. Darvell et al. also found that higher total mass losses
were obtained for the biomass fuels with higher volatile contents [103].
Fig. 31. Thermogravimetric mass curve (wt-% vs. temperature) of wet wood chip raw
material.
The TG mass curve of wood chip raw material is presented in Fig. 31. As earlier,
from the graph it can be observed that elevating the drying temperature first
extracts only water (moisture) from the wood chip material, which is indicated as
an almost linear weight decrease in the wood chip material mass curve. This
decrease in the mass of the wet wood chip material takes place until about a 120
°C drying temperature in the conditions of this study. These results are in
accordance with corresponding observations of Darvell et al. [103]. The next
sharp weight decrease in the mass curve indicates the drying temperature which
must not be exceeded during the drying process, because in that case the wood
chip material will catch fire. For the wet wood chip material this temperature is
71
achieved at about 230 °C. Based on the thermogravimetric mass curve results, the
maximum drying temperature of the wet wood chip material in this study is about
220 °C. On the other hand, it is observed from the graph (Fig, 31) that the
optimum drying temperature for wet wood chip material is about 120 °C, where
most of the moisture is evaporated but the wood chip raw material will not catch
fire and valuable volatiles are maintained. In addition, drying costs are low.
6.7 Particle size distribution (II)
It is generally known that the particle size of biomass and biomass-based
products, and therefore the particle size distribution of different materials, is
nowadays under wide study. So an important matter for the success of pelletizing
and understanding some physical and chemical properties of pellets is the particle
size of the materials used. For example, the optimal particle size of the raw
material affects the strength and processing properties of pellets. In this study the
milled raw material with a particle size of over 2000 µm was sieved off with a
Hosokawa Alpine Air Jet Sieve e200LS. The proportion of sieved material was
about 3.3% of the whole sample mass. Twenty mL of sample were taken for the
analysis.
Bergström et al. have indicated that particle size distribution has some effect
on current consumption and compression strength, but no evident effect on single
pellet and bulk density, moisture content, moisture absorption during storage and
abrasion resistance [104]. Differences in average total conversion time
determined for pellet batches tested under the same combustion conditions was
less than 5 % and not significant. The results are of practical importance,
suggesting that grinding sawdust particle sizes below 8 mm is probably needless
when producing softwood pellets. Thus, it seems that less energy could be used if
only oversized particles are ground before pelletizing [104].
In general, it has been observed that broad variation in particle size is best
with respect to pellet quality [105]. An overly high proportion of fine particles
(particle size with a diameter smaller than 0.5 mm) in the raw material has a
negative impact on both friction and pellet quality. The optimum particle size
depends on the densification process, for pellet production particles are usually
below 5 mm in diameter, depending on the material. As a rule of thumb, the
amount of fine materials should not exceed 10 to 20% unless a binding agent is
added [105].
72
The particle size distribution of milled wood (bark-free Scotch pine sawdust
and shavings) pellet raw material is presented in Fig. 32. Particle diameter in
micrometres is on the x-axis and volume weighted percentage (vol-%) is on the y-
axis. According to the particle size distribution plot of pellet raw material, it is
clear that the pellet wood material is very fine and quite homogeneous. For
comparison, the particle size distribution of corresponding Finnish wood ash from
the burning process is presented in Fig. 33. The results show that the particle size
distribution of wood ash is quite different from that of the raw material.
Fig. 32. Particle size distribution of milled pellet raw material (bark-free Scotch pine)
[Paper II].
73
Fig. 33. Particle size distribution of typical bio-ash from the burning process [106].
6.8 Wood-pellet-based bio-ash (II,IV,V)
The chemical and physical properties of two model wood-pellet-based bio-ashes
are described in this chapter. The corresponding results concerning the other
studied bio-ashes are quite similar and are presented in detail in papers II, IV and
V.
According to Table 5, the pH of the wood pellet ash sample 1 and 2 solutions
(wood pellet ash from Kuusamo and Jyväskylä) was 13.3 and 13.4, respectively,
and they were very strongly alkaline, in accordance with corresponding results for
other wood-based ashes [Papers IV,V,31,32,66,107]. This means they have a
strong liming effect. An alkaline pH indicates that at least part of the dissolved
metals in the ash occur as basic metal salts, oxides, hydroxides and/or carbonates
[65,66,67]. Thus, the proportion of soluble basic metal salts, oxides, hydroxides
74
and carbonates in the wood ash obviously outweighs the proportion of soluble
acidic components, and the wood pellet ash subsequently generates an alkaline
pH. Furthermore, according to the electrical conductivity value (58 mS cm-1)—an
index of the total dissolved electrolyte concentrations—the leaching solution of
the wood pellet ash has a relatively high ionic strength. This also indicates that at
least part of the dissolved metals occur as dissolved basic metal salts, e.g. oxides
and hydroxides. The dry matter content of wood pellet ash sample 1 was very
high (99.7%). From an environmental and health point of view, this can be
problematic because it may increase the amount of dust present during handling.
The dry matter content of ash sample 2 was also high, but a little lower (97.5%).
The slightly elevated LOI (5.3%; d.w.) and TOC (5.7%, d.w.) values of ash
sample 1 as well as the LOI (2.7%; d.w.) and TOC (4.0%, d.w.) of sample 2
indicate that samples 1 and 2 contain some unburned organic material due to
incomplete combustion of the fuel in the boiler. The easily soluble Ca
concentration of 100 g kg-1 (d.w.) in sample 1 was ca. 62.5 times higher than the
Ca concentration of 1.6 g kg-1 (d.w.) measured in typical coarse-textured mineral
soil in Finland. In sample 2 the corresponding ratio was 27. The concentrations of
easily soluble Mg (16.5 g kg -1; d.w. and 8.61 g kg-1; d.w.) and P (0.08 g kg-1; d.w.
and 0.02 g kg-1; d.w.) were correspondingly ca. 82.5 and 8 times higher in sample
1 and ca. 43 and 2 times higher in sample 2 than the typical value of Mg (0.2 g
kg-1; d.w.) and P (0.01 g kg-1; d.w.) in Finnish soil. The elevated Ca, Mg and P
concentrations in model wood pellet ash samples 1 and 2 indicate that wood-
pellet-based ash is also a potential agent for soil remediation and for improving
soil fertility. It would therefore be ecological beneficial if also wood-pellet-based
bio-ash could be returned to the forest ecosystem.
The relatively high, easily soluble concentrations of K in wood pellet ash
samples 1 and 2 (70 g kg-1 d.w. and 82 g kg-1 d.w., respectively), which contribute
to improving soil fertility, are also worth noting. In addition, it is worth noting
that the amounts of potassium alone in ash samples 1 and 2 obviously exceed the
limit value given in the new Ministry of Agriculture and Forestry Decree on
Fertilizer Products [56]. According to this decree the sum ratio of potassium (K)
and phosphorus (P) combined should be at least 2%, determined by microwave
assisted digestion using HNO3 as a digestion solution [53]. According to Tulonen
et al. [108], this kind of ash is especially recommendable for nitrogen-rich
peatlands, which suffer from a shortage of other nutrients.
The liming effect (neutralising value NV) is one of the most important
indicators in evaluating the agricultural value of ash, because ash acts as a liming
75
agent in acidic soil. The capacity of a liming agent to neutralise soil acidity
depends on its proportion of soluble and hydrolysable bases such as oxides,
hydroxides, carbonates and silicates. Cations such as calcium, magnesium and
potassium are interactive counter-ions. Liming capacity—the calculated
equivalence a neutralising material with a corresponding value of commercial
lime 38 [% Ca]—was calculated to be 1.1 t/t and 1.05 t/t, respectively, for wood
pellet ashes 1 and 2. These results mean that only 1.05–1.1 t of wood pellet ash is
needed to replace 1 t of commercial lime in liming applications, or the
neutralising value of the model pellet ashes is similar to that of commercial lime.
Omitting peat pellet ash, the corresponding values of liming capacity of the bio-
ashes studied in this work [Paper IV] changed from 1.05 to 3.6, indicating that all
the studied bio-ashes are potential neutralising agents for acidic soils.
Table 5. Physical-chemical properties and easily soluble nutrient concentrations in
wood pellet ash sample 1 from Kuusamo and wood pellet ash sample 2 from
Jyväskylä.
Parameter / nutrient Unit Wood pellet ash 1 Wood pellet ash 2
pH (1:5) --- 13.3 13.4
Electrical conductivity (EC) mS cm-1 58 10
Dry matter content (105 °C) % 99.7 97.5
Loss on ignition (LOI, 550 °C) % (d.w.) 5.3 2.7
Total organic carbon (TOC) g kg-1 (d.w.) 57 40
Neutralising value (NV) % (Ca; d.w.) 34 36
Reactivity value (R) % (Ca; d.w.) 26 25
Liming capacity a % (Ca; d.w.) 1,1 1,05
Ca g kg-1 (d.w.) 100 43
Mg g kg-1 (d.w.) 16.5 8.61
Na g kg-1 (d.w.) 5.0 21.9
K g kg-1 (d.w.) 70 82
S g kg-1 (d.w.) 6.7 4.4
P g kg-1 (d.w.) 0.08 0.02
Mn mg kg-1 (d.w.) 1370 597
Cu mg kg-1 (d.w.) 23 25
Zn mg kg-1 (d.w.) 350 177 a Equivalence with the neutralising value (38%) of commercial lime [t/t].
The total heavy metal concentrations in wood pellet ashes are expressed on a dry
weight (d.w.) basis. According to Table 6, the total heavy metal concentrations in
wood pellet ash 1 (Kuusamo) and wood pellet ash 2 (Jyväskylä) were lower than
76
the current Finnish limit values for maximal allowable heavy metal
concentrations in forest fertilizers [56]. On the other hand, the heavy metal
concentrations in both wood pellet ash 1 (Kuusamo) and wood pellet ash 2
(Jyväskylä) were higher than the current Finnish limit values for maximal
allowable heavy metal concentrations in agricultural use [56], but only for
cadmium. It should be noted that the old limit values are presented and discussed
in papers IV and V. In Finland the limit values in agricultural use have stayed the
same, but the new limit values for heavy metal concentrations in fertilizers used
in forestry came into force in September 2011 [56]. In this context it is worth
noting that we did not determine e.g. the toxicity of the wood pellet ash. This is
due to the fact that, according to Finnish legislation, information about the
toxicity is not necessary when ash is utilised. However, in many cases the
competent authority may decide that toxicity has to be determined with toxicity
tests before the ash can be utilised as fertilizer. A comprehensive review of the
Finnish limit values and legislation concerning the use of materials as forest
fertilizers in Finland is given in the new Ministry of Agriculture and Forestry
Decree on Fertilizer Products 24/11 [56].
Table 6. Total heavy metal concentrations (mg kg-1; d.w.) in wood pellet ash sample 1
(Kuusamo) and wood pellet ash sample 2 (Jyväskylä) and the current Finnish limit
values (mg kg-1; d.w.) for wood-, peat- and biomass-derived ashes used as forest
fertilizer and in agricultural use, determined by microwave-assisted HNO3 acid
digestion [55].
Metal Wood pellet ash 1 Wood pellet ash 2 Limit value
forest fertilizer
Limit value
agricultural use
Cd 16 9 25 2.5 a
Cu 160 180 700 600
Pb 12 15 150 100
Cr 180 84 300 300
Zn 640 360 4500 1500
As < 3.0 < 3.0 40 25
Ni 110 30 150 100
Hg < 0.03 < 0.03 1.0 1.0
a For ash-based fertilizer.
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Solubility results for different heavy metals in 3-stage BCR extraction in wood
pellet ash sample 1 are presented as histograms in Fig. 34, in which the presented
total concentrations are determined by microwave-assisted acid digestion (HCl +
HNO3) [53]. Solubility results of different heavy metals in 3-stage BCR
extraction in wood pellet ash sample 2 are presented as histograms in Fig. 35. The
percentages of metals presented in the figures and the BCRtotal values can be
changed into metal concentrations. Furthermore, potential bioavailabilty
percentages PBPM can be calculated with Eq. (12) and are given for the different
metals in Figs. 34 and 35.
Fig. 34. Distribution of heavy metals in wood pellet ash sample 1 (Kuusamo) in 3-stage
BCR extraction between exchangeable (BCR S1), easily reduced (BCR S2) and
oxidizable (BCR S3) fractions.
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Fig. 35. Distribution of heavy metals in wood pellet ash sample 2 (Jyväskylä) in 3-
stage BCR extraction between exchangeable (BCR S1), easily reduced (BCR S2) and
oxidizable (BCR S3) fractions.
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6.8.1 Utilisation potential of bio-ash
As presented earlier, as part of the EU and Finnish national waste strategies, the
primary goal also for bio-ash is its utilisation as material. Therefore, there is
currently tremendous interest in the EU countries in substantially increasing the
utilisation of bio-ash, developing new applications and developing new bio-ash-
based products, especially granulated bio-ash-based products. Many factors, such
as combustion technology, affect the properties of bio-ashes. As in the case of
wood pellets, binding agents can slightly affect the composition and properties of
ash. For example, when lignosulphonate is used as a binding agent, it slightly
increases the sulphur content of pellets [Paper II]. However, it is generally known
that wood biomass is very pure bioenergy and thus the results obtained in this
thesis work support the assumption that wood and wood-based pellet ash have
great utilisation potential and they are suitable for use in new eco-, cost- and
material-efficient applications even in the near future as part of extensive bio-ash
utilisation.
Granulated ash-based products in particular seem to have great potential. Ash
can also be pelletized, but it is not economically profitable, contrary to granulated
ash products. Lately there are many studies in progress or under consideration
concerning this area [109]. In addition to agricultural and forestry fertilizers, there
are also several other utilisation applications for wood-based bio-ashes, such as
liming and soil conditioning, road construction and landscaping, combustion, as
adsorbents in industrial and municipal waste water treatment, in concrete industry
and in new applications [106]. One of the new applications for granulated ash is
polymeric geo-ash, which can be used for manufacturing different geo-polymers.
Utilisation of steel slag—a chemically pure, voluminous and thus economical
industrial by-product—mixed with bio-ash has also been under study [32]. There
have also been promising results in using granulated ash to remove phosphorus in
waste water treatment. Because wood-based pellet ashes are strongly alkaline, it
can be assumed that they can be used for heavy metal removal in waste water
treatment applications. It is noteworthy to point out that the new ash-based
environmentally friendly products can also have utilisation potential at the global
level. In this thesis work only pine-based ash was studied, but it would be
interesting to also examine ashes from other wood species.
The results of sequential leaching of wood-pellet-based bio-ash in this thesis
show, in agreement with earlier studies on other ashes, such as bottom or fly ash,
paper mill sludge, green liquor dregs, PCC waste, lime waste from pulp and paper
83
mills and waste rock material originating from a zinc mine
[65,66,67,68,69,70,71,72], that conditions have a large effect on the solubility of
all heavy metals, and therefore on their mobility, bioavailability and
environmental risk. In addition, the total concentration of every element is much
larger than its solubility in each potential bioavailability fraction, because the
highest concentrations of all metals occurred in the residual fractions; see
equations (13) and (14). Thus, the total concentrations of heavy metals, which
generally have been used in the legislative procedure in e.g. utilisation of bio-ash
for agricultural and forest fertilizers [56] and in assessment of soil contamination
and remediation needs [58], is a poor measure of real environmental risk.
Sequential leaching provides such information for utilisation purposes, which is
necessary if we wish to know the real environmental risk of metals in different
conditions possible in natural conditions now or in the future, i.e. not only in
terms of the conditions related to permit applications.
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7 Conclusions
Using Finnish wood, mainly Scotch pine (bark-free sawdust and shavings) as a
model raw material, the total functionality of a pilot-scale pellet facility combined
with an extensive chemical toolbox was tested and examined in practice in this
thesis work. The chemical toolbox includes measurements of moisture content,
density, heat value, mechanical durability and particle size distribution, TG
analysis and elementary analysis as well as new methods for pellet biodegradation
using the BOD OxiTop method and an optical microscopic staining method. To
improve the quality of pellets, considering the profitability of production and
occupational safety factors, it is practicable to use binding agents, and to
characterise them a new specific optical microscopic staining method using
starch-containing binding agents was developed and tested in this thesis work.
The aim in the future is to expand the base of wood pellet raw material by
utilising more low-value and/or moist biomass to, especially logging residue from
thinning. For example, if only 10–15% of unusable waste wood biomass from
forestry operations could be utilised, it would raise annual wood pellet production
capacity significantly, being about 2–3 million tonnes. The purchase prices of
poor-quality biomasses are very low, sometimes almost free, so it doesn’t increase
the total cost of pellet production significantly, which makes them economically
beneficial and interesting.
In pellet production the manufacturers normally dry the raw material to a
water content of 8–12% before the pelletizing process begins. Optimum pellet
quality is obtained with a sawdust moisture content in the range of 11–13%. In
experiments, wet sawdust (moisture content about 50%) needed to be dried 60
minutes to achieve the required pelletizing dryness and fresh wood chips
(moisture content about 50%) needed to be dried 150 minutes. For peat the
required pelletizing moisture content was achieved in 300 minutes when the
drying temperature was 105 °C.
Binding agents lignosulphonate, residual potato flour or potato peel residue
did not have any significant effect on the calorimetric heat value of the wood-
based pellets, due to the low content of the binding material in the pellets and the
differences between the heat values of the studied wood and binding materials.
Additives or binding agents, < 2% of total mass, have only little effect on
combustion. Even if the maximum amount of a high-energy-content additive were
used, the total effect would be a less than 3% increase in heating value.
86
High biodegradability of raw materials, pellets and binding agents may cause
considerable economic losses in wood pellet production during storage and
transportation and may also cause health problems in pellet warehouses.
However, the concentrations of hazardous volatile compounds (carbon monoxide,
hexanal) are low when pellet biodegradation does not take place. Potato peel
residue itself has high biodegradability, but it seems to even slightly decrease the
biodegradation of pellets when it is used as a binding material. The effect can be
explained by the prevention of oxygen gas adsorption into pellet pores. Pellets
containing lignosulphonate, potato peel residue and residual potato flour
measured in the solid phase did not degrade at all. The BOD reactions in the
studied wood-based pellets do not have any practical effect on their mechanical
durability, nor do they cause mass loss even during a relatively long period of
storage. However, various fungi and other micro-organisms decompose moist
wood in the solid phase. The content of condensate water from drying of Finnish
wood chips is biodegradable and purification of condensate waters of wood chips
is spontaneous, and thus condensate water from drying is pure and its treatment
causes neither problems nor costs.
Binding agents or additives may be added to improve the pellet’s mechanical
properties, for example to increase strength and density, improve pelletizing
throughput or improve moisture resistance. From the environmental, economic
and logistical points of view binding materials should be industrial by-products
and locally utilisable residuals. For example, logging residue from thinning has
great potential for utilisation when expanding the raw material basis of
pelletizing, but usable binding agents are needed for low-value and moist
biomasses. Lignosulphonate, residual potato flour and potato peel residue were
used and tested as model adhesive binding agents. All three tested binding
materials had a positive effect on pellet compactness. Lignosulphonate seems to
have a lubrication effect, which reduced power consumption of the pellet press
during experiments. Lignosulphonate also raised the production rate of pellets,
but the amount must be ≤ 0.5% in order to keep the sulphur concentration of the
pellets below the normative limit. By using potato peel residue as a binding
material it is possible to prevent harmful pellet biodegradation. Adding potato
flour does increase power consumption, but it also improves pellet quality while
having no noticeable effect on inorganic content.
In the microscope images based on the optical microscopic staining method,
starch compounds appear as small dark spots and areas. Microscope images
enable us to analyse the binding structure of the pellet and starch penetration. This
87
characterisation method was proven to function selectively and quantitatively
correctly for starch compounds in terms of binding agent content examination.
The poor quality of pellets is also visible from optical microscopic images, as the
effect of pellet quality (transparency and irregularity of shape) can be visually
observed from the images. In the future, computational image analysis will be
integrated into the analysis procedure. In conclusion, the optical microscopic
staining method was shown to be a useful method for characterisation of starch-
containing binding agents in wood pellets. The information obtained with the
staining method and microscopic structure analysis can be used for planning the
feeding of binding materials in pellet plants (place, amount) and also for
optimizing the pelletizing process.
From thermogravimetric (TG) curves it is possible to estimate the mass
change of different compounds during drying. The information received can then
be used to optimize the drying temperature for different types of wet biomaterials.
For modeling the drying process and optimizing the drying temperature TG
measurements were carried out for two different wood pellet raw materials: wet
sawdust and wood chips. The maximum drying temperature of the wet sawdust in
this study is about 200 °C and the optimum drying temperature for sawdust is
about 100 °C, when most of the moisture is evaporated, but volatile compounds
with high heat value are not lost and drying costs are low. The maximum drying
temperature of the wet wood chips raw material is about 220 °C and the optimum
drying temperature for wood chips is about 120 °C. TG analysis was shown to be
useful method for optimizing and modeling the drying process of different
biomass raw materials for pelletizing.
Particle size distribution has some effect on current consumption and
compression strength but no effect on single pellet and bulk density, moisture
content, moisture absorption during storage and abrasion resistance. Grinding of
sawdust particle sizes below 8 mm is not needed when producing softwood
pellets. The optimum particle size depends on the biomass densification process,
for wood pellet production particles are usually below 5 mm in diameter,
depending on the wood material used. The presence of fine materials (particles
smaller than 0.5 mm in diameter) has a negative impact on pellet quality. The
amount of fine materials should not exceed 10 to 20% unless a binding agent is
added.
The results show that only 1.05–1.1 t of wood pellet ash is needed to replace
1 t of commercial lime in liming applications. Wood-pellet-based ash is also a
potential agent for soil remediation and for improving soil fertility. It is worth
88
noting that the amounts of potassium alone in wood ash samples obviously
exceed the limit value given in the new Ministry of Agriculture and Forestry
Decree on Fertilizer Products, since according to this decree the sum ratio of
potassium (K) and phosphorus (P) combined should be at least 2 %. Because
wood biomass is very pure bioenergy, wood and wood-based pellet ash have great
utilisation potential and they are suitable for use in new eco-, cost- and material-
efficient applications in the future. Granulated bio-ash-based products in
particular have great potential. There are several potential utilisation applications
for these wood-based bio-ashes, such as agricultural and forestry fertilizers,
liming and soil conditioning, road construction and landscaping, combustion,
concrete industry and other new applications, but above all they have economic
potential as adsorbents in industrial and municipal waste water treatment.
89
8 Future work
The aim of multidisciplinary pellet research in the future is to widely utilise non-
utilised low-value and/or moist biomass, especially logging residue from
thinning, and thus substantially expand the base of wood pellet raw materials.
Because these raw materials are mainly situated in rural areas, this decentralised
pellet production will also have an employing effect in these areas. Using short
rotation tree species, such as willow, aspen, alder and eucalyptus as well as other
tree species, e.g. spruce and birch, as a wood pellet raw material in pelletizing
needs to be examined and tested in more detail. Also other biomasses and
industrial by-products in Finland have potential for utilisation as pellets; for
example, agricultural materials such as various straws (barley, oat, wheat, grass),
peat and reed canary grass. A negative aspect with these materials is, however, the
problems in combustion that arise because of the large amount of ash, which is
usually even 5–10%.
In the present research the aim was to find eco- and cost-efficient binding
materials and thus improve the competitiveness of pellet products. Therefore,
various locally tailored binding agents such as certain industrial by-products and
residuals, especially starch-containing waste materials and lignin, are currently
under study and their usage potential needs to be researched further in more
detail. Industrial use of potato peel residue as a pellet binding agent and in many
other applications is not possible in practice without functional drainage, because
of its rapid biodegradation and anaerobic decomposition. Thus, development of
cost-efficient methods, e.g. enzymatic or mechanical drainage of this waste
fraction, is under study for many purposes. One of the new binding agent
applications that will require further research is enzymatically and mechanically
processed fibre, so called zero fibre, which is a by-product of the chemical pulp
industry.
In the future, computational image analysis will be integrated into optical
microscopic staining procedure analysis. Further research based on microscopic
images concerning pellet quality is needed. The same kind of functioning
selective characterisation method has to be developed also for other binding
agents, such as lignosulphonate.
Wood-based pellet ash as a pure product has great utilisation potential and is
suitable for use in new eco-, cost- and material-efficient applications in the future
as part of extensive ash utilisation. Granulated ash products in particular will be
of great interest. Only pine-based wood pellet ash was studied in this thesis work,
90
but in the future it would be interesting to examine pellet ashes also from other
tree species, such as hardwood (birch) and short rotation tree species (willow) as
well as ashes from agricultural raw materials.
From the environmental, energy-related and social (employing effect) points
of view the goal is to develop decentralised pellet production alongside of large-
scale pellet factories, so that pellet production will have more essential
importance in energy policy than it does today. Therefore, much more
multidisciplinary research is needed to achieve this goal, for which purpose the
developed chemical toolbox is a very practicable tool.
91
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Original papers
I Kuokkanen M, Kuokkanen T, Stoor T, Niinimäki J & Pohjonen V (2009) Chemical Methods in the Development of Eco-efficient Wood-based Pellet Production and Technology, Waste Management & Research 27: 561–571.
II Kuokkanen M, Vilppo T, Kuokkanen T, Stoor T & Niinimäki (2011) Additives in Wood Pellet Production – A Pilot-Scale Study of Binding Agent Usage, BioResources 6 (4): 4331–4355.
III Kuokkanen M, Prokkola H, Larkomaa J, Stoor T, Siltaloppi L & Kuokkanen T (2010) Specific Staining and Optical Microscopy – a New Method for Characterisation of Starch-containing Wood Pellets, Special Issue of Research Journal of Chemistry and Environment, Proceedings of ICCE-2009: 311–317.
IV Kuokkanen M, Kuokkanen T, Nurmesniemi H & Pöykiö R (2009) Wood pellet ash – a potential forest fertilizer and soil conditioning agent (a case study), The Journal of Solid Waste Technology and Management, Proceedings in ICSW 2009: 659–667.
V Kuokkanen M & Kuokkanen T (2009) Puu- ja turvepellettien sekä hakkeen lämpökeskus- ja pienpoltossa syntyvien tuhkien hyötykäyttöön liittyvä tutkimusraportti, University of Oulu, Report Series in Chemistry, Report No. 74.
Reprinted with permission from SAGE Publications (I), BioResources (II),
Research Journal of Chemistry and Environment (III), Widener University School
of Engineering (IV) and Oulu University Press (V).
Original publications are not included in the electronic version of the dissertation.
100
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UNIVERS ITY OF OULU P.O.B . 7500 F I -90014 UNIVERS ITY OF OULU F INLAND
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SCIENTIAE RERUM NATURALIUM
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SCIENTIAE RERUM SOCIALIUM
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EDITOR IN CHIEF
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Senior Assistant Jorma Arhippainen
University Lecturer Santeri Palviainen
Docent Hannu Heusala
Professor Olli Vuolteenaho
University Lecturer Hannu Heikkinen
Director Sinikka Eskelinen
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Professor Olli Vuolteenaho
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ISBN 978-952-62-0103-0 (Paperback)ISBN 978-952-62-0104-7 (PDF)ISSN 0355-3191 (Print)ISSN 1796-220X (Online)
U N I V E R S I TAT I S O U L U E N S I SACTAA
SCIENTIAE RERUM NATURALIUM
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SCIENTIAE RERUM NATURALIUM
OULU 2013
A 607
Matti Kuokkanen
DEVELOPMENT OF AN ECO- AND MATERIAL-EFFICIENT PELLET PRODUCTION CHAIN—A CHEMICAL STUDY
UNIVERSITY OF OULU GRADUATE SCHOOL;UNIVERSITY OF OULU,FACULTY OF SCIENCE,DEPARTMENT OF CHEMISTRY
A 607
ACTA
Matti K
uokkanen